add workflow_dispatch event trigger for cleaning up cache by branch

[atravitz]

I'd like to have this for when we know we need to clear a cache to address test failures on a specific PR.

Checklist

• Added a news entry

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• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[github-actions]

No API break detected ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.89%. Comparing base (932f9f0) to head (cb7b0da).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1644      +/-   ##
==========================================
- Coverage   95.34%   92.89%   -2.45%     
==========================================
  Files         183      183              
  Lines       15779    15779              
==========================================
- Hits        15044    14658     -386     
- Misses        735     1121     +386     

Flag	Coverage Δ
fast-tests	92.89% <ø> (?)
slow-tests	?

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[mikemhenry]

Sounds good, also makes it easy to test it if we need to make changes, BTW, this will also let you clear the chache as well:
https://github.com/OpenFreeEnergy/openfe/actions/caches