Use the github actions from sigstore/scaffolding.

[vaikas]

Signed-off-by: Ville Aikas vaikas@chainguard.dev

Summary

Remove explicit sigstore/scaffolding installations + tests now that sigstore/scaffolding has a github action that we can use instead.

Ticket Link

Fixes

Release Note

[codecov-commenter]

Codecov Report

Merging #1699 (405dbe0) into main (b099136) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1699      +/-   ##
==========================================
- Coverage   29.43%   29.37%   -0.06%     
==========================================
  Files         141      141              
  Lines        8413     8429      +16     
==========================================
  Hits         2476     2476              
- Misses       5668     5684      +16     
  Partials      269      269              

Impacted Files	Coverage Δ
pkg/cosign/tlog.go	4.31% <0.00%> (-0.24%)	⬇️
cmd/cosign/cli/verify/verify_blob.go	10.49% <0.00%> (-0.14%)	⬇️
pkg/cosign/verify.go	18.45% <0.00%> (+0.03%)	⬆️

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