Add pixi support and migrate CI from conda to pixi to fix CI

[traversaro]

The conda CI had a bunch of old workarounds that probably resulted in all the errors in #1021 . In this PR, the workarounds are removed and pixi support was added, and CI has been migrated from conda to pixi.

The pixi-based workflows has some advantages:

• Reproducibility, as CI on PRs uses the lock file, while regression against dependency updates is tested in scheduled jobs (that delete the lock file)
• Easy to reproduce locally, as you just need to clone the repo and run pixi run test
• Speed up in the CI and in iterative local builds, thanks to ccache use and the use of the fast mold linker

[traversaro]

mold on macOS is failing with error:

2026-03-23T07:51:30.1213390Z -- Check for working C compiler: /Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/bin/arm64-apple-darwin20.0.0-clang
2026-03-23T07:51:31.6313830Z -- Check for working C compiler: /Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/bin/arm64-apple-darwin20.0.0-clang - broken
2026-03-23T07:51:31.6315070Z CMake Error at .pixi/envs/default/share/cmake-4.3/Modules/CMakeTestCCompiler.cmake:67 (message):
2026-03-23T07:51:31.6315600Z   The C compiler
2026-03-23T07:51:31.6315950Z 
2026-03-23T07:51:31.6316480Z     "/Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/bin/arm64-apple-darwin20.0.0-clang"
2026-03-23T07:51:31.6317210Z 
2026-03-23T07:51:31.6317390Z   is not able to compile a simple test program.
2026-03-23T07:51:31.6318120Z 
2026-03-23T07:51:31.6318460Z   It fails with the following output:
2026-03-23T07:51:31.6319440Z 
2026-03-23T07:51:31.6320000Z     Change Dir: '/Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.build/CMakeFiles/CMakeScratch/TryCompile-xft0tF'
2026-03-23T07:51:31.6320620Z     
2026-03-23T07:51:31.6321280Z     Run Build Command(s): /Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/bin/ninja -v cmTC_920b7
2026-03-23T07:51:31.6324700Z     [1/2] ccache /Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/bin/arm64-apple-darwin20.0.0-clang   -ftree-vectorize -fPIC -fstack-protector-strong -O2 -pipe -isystem /Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/include  -arch arm64 -isysroot /Applications/Xcode_16.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -MD -MT CMakeFiles/cmTC_920b7.dir/testCCompiler.c.o -MF CMakeFiles/cmTC_920b7.dir/testCCompiler.c.o.d -o CMakeFiles/cmTC_920b7.dir/testCCompiler.c.o -c /Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.build/CMakeFiles/CMakeScratch/TryCompile-xft0tF/testCCompiler.c
2026-03-23T07:51:31.6329100Z     [2/2] : && /Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/bin/arm64-apple-darwin20.0.0-clang -ftree-vectorize -fPIC -fstack-protector-strong -O2 -pipe -isystem /Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/include  -arch arm64 -isysroot /Applications/Xcode_16.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -Wl,-headerpad_max_install_names -Wl,-dead_strip_dylibs -Wl,-rpath,/Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/lib -L/Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/lib  -fuse-ld=mold CMakeFiles/cmTC_920b7.dir/testCCompiler.c.o -o cmTC_920b7   && :
2026-03-23T07:51:31.6331640Z     FAILED: [code=1] cmTC_920b7 
2026-03-23T07:51:31.6334690Z     : && /Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/bin/arm64-apple-darwin20.0.0-clang -ftree-vectorize -fPIC -fstack-protector-strong -O2 -pipe -isystem /Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/include  -arch arm64 -isysroot /Applications/Xcode_16.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -Wl,-headerpad_max_install_names -Wl,-dead_strip_dylibs -Wl,-rpath,/Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/lib -L/Users/runner/work/bipedal-locomotion-framework/bipedal-locomotion-framework/.pixi/envs/default/lib  -fuse-ld=mold CMakeFiles/cmTC_920b7.dir/testCCompiler.c.o -o cmTC_920b7   && :
2026-03-23T07:51:31.6337990Z     arm64-apple-darwin20.0: error: invalid linker name in argument '-fuse-ld=mold'
2026-03-23T07:51:31.6338400Z     ninja: build stopped: subcommand failed.

so even if mold package is available for mac, I guess it is not actually supported. I guess probably mold is available as a binary in mac, but does not support Mach-O format, so it does not make sense to try to use, let me disable it in macos.

[traversaro]

Somehow (as discussed in #992) the Windows test are still failing. As this is unrelated to this PR, I will skip again Python tests on Windows.

[traversaro]

@GiulioRomualdi the PR is ready for review, the only remaining failure is #1029 (that I would strongly suggest to just drop).

[GiulioRomualdi]

Thank you @traversaro! We can merge it

[traversaro]

Thank you @traversaro! We can merge it

Thanks!