fix: mergebom cosign verification

[dervoeti]

Fix for #827

Sorry, I hope this is the last fix: I built mergebom as a container image first but then switched to building a binary and forgot to adapt the signature check in the GitHub action, so that it matches the new workflow name.

Tested the change locally, works:

curl -L -o mergebom https://repo.stackable.tech/repository/packages/mergebom/stable-x86_64

curl -L -o mergebom.bundle https://repo.stackable.tech/repository/packages/mergebom/stable-x86_64_signature.bundle

cosign verify-blob --certificate-identity 'https://github.com/stackabletech/mergebom/.github/workflows/build_binary.yaml@refs/heads/main' --certificate-oidc-issuer https://token.actions.githubusercontent.com --bundle mergebom.bundle mergebom
Verified OK