fix goreleaser signning part

[cpanato]

see https://github.com/sigstore/policy-controller/actions/runs/12599069304/job/35116178539

to fix that we set the paralellism to 1, we did this in other pipelines

[vaikas]

Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.77%. Comparing base (50ef092) to head (f944765).
Report is 301 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1753       +/-   ##
===========================================
- Coverage   52.92%   42.77%   -10.16%     
===========================================
  Files          44      121       +77     
  Lines        3979     8987     +5008     
===========================================
+ Hits         2106     3844     +1738     
- Misses       1651     4788     +3137     
- Partials      222      355      +133     

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