Artifact upload step runs even when provenance generation fails

[deobieta]

The artifact upload step in the compute_slsa_source workflow runs even when the provenance generation step fails, resulting in unnecessary warnings and potential confusion.

In this workflow run

The provenance generation step failed with:
Error: commit c167f7680ab3a34a6e60135eaaa8284fb2c948fe has more than one parent ([...]), which is not supported ##[error]Process completed with exit code 1.

However, the actions/upload-artifact step still executed:

- uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02  # v4.6.2
   if: always()
      with:
        name: prov_metadata
        path: ./metadata/

This resulted in the warning:
! No files were found with the provided path: ./metadata/. No artifacts will be uploaded.

The artifact upload step should be conditional and only run when the provenance generation step succeeds. This would avoid unnecessary step execution and make the failure clearer to users.