[CI] Tune nightly benchmarking job for better reliability

[ianayl]

This PR tunes the nightly benchmarking job to produce more consistent results:

• Lowers the tolerance threshold of benchmarking results accepted from 50% to 8%
  • Nightly was flaking before even with a 50% tolerance threshold
• Raises the iterations to 5000
  • Using 10,000 iterations did not result in significantly more stable performance, although this may change as we obtain more data
  • However, the PVC benchmarking job in the overall nightly workflow now takes about ~47 minutes, whereas before the PVC benchmarking job took ~14 minutes
    • This should not have major impact on execution time however, considering the E2E tests take ~42 minutes: Since both these jobs run in parallel on different machines, the theoretical effect on the overall workflow should only be about 5 minutes, although this would depend on whether or not machines are able to be scheduled in time.
• Changes the benchmarking workflows in sycl-nightly.yml to use the tuned PERF_PVC runner
  • Untuned machines are exhibiting large variations when running compute-benchmarks (20-25%, up to 50% in the worst case scenario): These are unacceptable variations and not particularly useful.
• Disables nightly benchmarking on gen12:
  • Gen12 machines are currently untuned. Similar to PVC machines, these results are not accurate and not worth serious nightly benchmarking.
• Adds guards for benchmarking jobs to prevent benchmark runs in forks [CI] Add CI workflow to run compute-benchmarks on incoming syclos PRs #14454 (comment)

[sarnex]

minor comments

[sarnex]

should we make it a shared action in devops/actions if its called by UR CI also?

[ianayl]

When we combine both CI's there should only be 1 invocation of this, but that's when the dust all settles

[ianayl]

@intel/llvm-gatekeepers PR ready for merge, thanks!