[CI] Move SPIRV-backend testing to post-commit

[aelovikov-intel]

Development happens mostly upstream, so there is no need to have that in pre-commit, post-commit should be enough.

Before this PR, when running E2E tests in "in-tree" mode (as opposite to a standalone e2e tests cmake invocation), some utilities (e.g. llvm-ar) were only searched for in PATH that didn't contain freshly built toolchain. Our pre-commit used a container image with last successful "Nightly" toolchain installed, so those were picked up from there. Post-commit uses GNU Compiler instead of Clang, so these utilities aren't available in PATH.

This PR modifieslit.cfg.py such that we search for those utilities in the newly built toolchain first (still not modifying PATH which seems to be the approach taken in LIT). That also ensures that opencl-aot feature is available when running E2E tests in build-only mode in Post-commit's build job.

[aelovikov-intel]

Ping @againull , @reble , @intel/sycl-graphs-reviewers .

[againull]

Initially I assumed this was already approved on behalf of llvm-reviewers-runtime

[sarnex]

Yeah sorry I'm not really a runtime team member so I usually re-add the team to reviews whenever my review is used for runtime, I'm in the group for CI/test reasons

[againull]

Yeah sorry I'm not really a runtime team member so I usually re-add the team to reviews whenever my review is used for runtime, I'm in the group for CI/test reasons

No worries, that's my bad

[aelovikov-intel]

@reble , @intel/sycl-graphs-reviewers , ping. The change in the graphs tests is minimal and shouldn't take more than a few minutes to review.

[adamfidel]

LGTM. Apologies for the slow review.

[aelovikov-intel]

Windows failure is #19660, unrelated.