security workflow: use a verbatim concurrency groupUnverified
rickub resolves every concurrency group that mentions github.ref to the
bare ref, so security-${{ github.ref }} and ci.yml's ci-${{ github.ref }}
became one group and the Security run cancelled CI #15 on the same push.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FgkcT3UPxwxukBKojB5eg4