Parallelize x64 boot tests with matrix jobs

[jserv]

This moves boot tests from host-x64 job to dedicated boot-tests-x64 job with matrix strategy for parallel execution:

• gcc: interpreter, jit (2 parallel jobs)
• clang: interpreter, jit, t2c (3 parallel jobs)

T2C boot test runs only on clang since it uses LLVM, eliminating redundant testing on gcc.

boot-tests-x64 job runs independently, allowing boot tests to execute concurrently with other CI jobs. It also increases boot-tests-x64 job timeout to provide adequate buffer.

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Summary by cubic

Parallelizes x64 boot tests into a dedicated matrix job to cut CI wall-clock time. Boot tests now run alongside other jobs with per-mode timeouts, and CI also triggers on workflow file changes.

• Refactors
  • Moved boot tests from host-x64 to a new boot-tests-x64 matrix: gcc {interpreter, jit} and clang {interpreter, jit, t2c}.
  • Limited T2C to clang; installs and caches LLVM for all clang builds (needed for LTO) and verifies t2c.o to ensure LLVM detection.
  • Added per-mode BOOT_TIMEOUT and increased job timeout; cache keys include compiler/boot_type to isolate artifacts.
  • Kept architecture tests in host-x64; boot-tests-x64 fetches Linux artifacts and runs independently with fail-fast disabled.
  • Included .github/workflows/** in code change detection so CI runs on workflow-only changes.

^{Written for commit a053528. Summary will update on new commits.}

[cubic-dev-ai]

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