workflows/triage: fix concurrency.
workflows/triage: fix concurrency.

`github.ref` is always `master` for `pull_request_target` jobs so this
needs to use `github.head_ref` instead to get the actual pull request
branch.

MikeMcQuaid/brew,nandub/brew,nandub/brew,MikeMcQuaid/brew,JCount/brew,Homebrew/brew,JCount/brew,MikeMcQuaid/brew,Homebrew/brew,Homebrew/brew,nandub/brew,JCount/brew,nandub/brew,MikeMcQuaid/brew,JCount/brew,Homebrew/brew