ci: check for signed commits

[marijanp]

No description provided.

[koxu1996]

Suggestion: We can run commit checks independently, so developers could see if build-test works, even if they forget to sign the commits.

@marijanp What about adding separate workflow, e.g. .github/workflows/pre-check.yml:

name: Check signed commits in PR 
on: pull_request_target

jobs:
  check-signed-commits:
    name: Check signed commits in PR
    runs-on: ubuntu-latest
    permissions:
      contents: read
      pull-requests: write
    steps:
      - name: Check signed commits in PR
        uses: 1Password/check-signed-commits-action@v1