[ci] separate output directory for different fixtures

[Jiahui17]

Previously, we always reused the same output directory (e.g., fir/out/...) for different test fixtures (e.g., speculation vs. sharing) on the same benchmark, it has two issues:

1. When tests are parallelized, there might be a race condition between these separate tests on the same benchmark
2. Github action only uploads artifact for the last run. This is problematic for troubleshooting

This PR deduplicates the output directory for each test fixture

[Jiahui17]

The generated name is a bit long, but this is just for debugging, so it should be fine