Use commit SHAs instead of refs in benchmark workflow

[lola831]

Description

Updated the benchmark workflow to use commit SHAs instead of refs when checking out the base and compare versions. This makes the benchmark run against specific commits rather than moving branch refs.

Issue Link

Checklist

• I have tested the changes locally via pytest and/or other means
• I have added or updated relevant documentation
• I have autoformatted the code with black and isort
• I have added test cases (if applicable)

Additional Notes

[dfremont]

Good idea, thanks Lola!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.66%. Comparing base (3c5102e) to head (ffadf6e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #456      +/-   ##
==========================================
- Coverage   89.71%   89.66%   -0.05%     
==========================================
  Files          48       48              
  Lines       13194    13194              
==========================================
- Hits        11837    11831       -6     
- Misses       1357     1363       +6     

see 3 files with indirect coverage changes

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