ci/cd: sign commits from goreleaser

[kamilsk]

Motivation: support confidence.

Research:

• https://httgp.com/signing-commits-in-github-actions/
• https://github.community/t/sign-bot-commit-in-actions/17896

Related issues:

• kamilsk/homebrew-tap/issues/8
• org: rename main branch and take a snapshot octolab/homebrew-tap#3