Use cosign @ HEAD for Github OIDC sign blob test

[priyawadhwa]

github oidc run is still failing because it doesn't install cosign @ HEAD, so it doesn't have fix we merged today.

Signed-off-by: Priya Wadhwa priya@chainguard.dev

Summary

Ticket Link

Fixes

Release Note

[codecov-commenter]

Codecov Report

Merging #1678 (299f37b) into main (e596625) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1678      +/-   ##
==========================================
- Coverage   29.29%   29.25%   -0.04%     
==========================================
  Files         140      140              
  Lines        8370     8370              
==========================================
- Hits         2452     2449       -3     
- Misses       5652     5654       +2     
- Partials      266      267       +1     

Impacted Files	Coverage Δ
pkg/cosign/tuf/client.go	62.34% <0.00%> (-0.95%)	⬇️

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