Allow libclang_scanner_scan_dependencies to propagate an environment dictionary to clang

[jakepetroules]

libclang_scanner_scan_dependencies is invoked in-process and doesn't currently have a means to propagate an environment dictionary, so it uses the calling process's.

This impacts the explicitModulesEnvironmentTest(), which has to mutate the process environment as a workaround.

[jakepetroules]

Turns out this has nothing to do with llbuild (we're already doing the correct thing there). Updated the title/description.