fix: fix method for getting leaf certs in Bundle v0.3

[ramonpetgrave64]

Followup to slsa-framework/slsa-github-generator#3777

This PR adds a missing modification for getting the leaf certificate in the new Bundle format v0.3.

In my original experiments, I did have this method in a dev branch, but neglected to include it in the final PR.

• main...verify-sigstore-go-Bundlev3#diff-a9bfffae1bd0d145e950805e7a35b8e65adc7a68affa605b484f4831097b989cR98-R107
• https://github.com/slsa-framework/slsa-verifier/pull/799/files

Testing

• I re-used the same attestation file from a failing workflow for unit tests and manual invocation.
  • https://github.com/slsa-framework/example-package/actions/runs/11511156484

Followup

• Finish finding a way to test changes within PRs.
  • feat: produce sigstore Bundles for generic generator and go builder workflows slsa-github-generator#3777 (comment)
  • feat: testing mode from non-main slsa-framework/slsa-github-generator branches #797

[ramonpetgrave64]

@haydentherapper @loosebazooka