Revert "ci: pin slsa-framework action to sha"

[tobyhawker]

Reverts #2610

According to slsa documentation, only tags are supported: https://github.com/slsa-framework/slsa-github-generator/blob/main/README.md#referencing-slsa-builders-and-generators

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.67%. Comparing base (7f7d8c3) to head (6aee4d4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2678      +/-   ##
==========================================
+ Coverage   71.63%   71.67%   +0.03%     
==========================================
  Files         164      164              
  Lines       12080    12080              
==========================================
+ Hits         8654     8658       +4     
+ Misses       2796     2794       -2     
+ Partials      630      628       -2     

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