.github: Change concurrency group ids so they're unique.

GitHub strikes again.  Apparently the github.ref context variable only
contains the PR number if the workflow is triggered by "pull_request" so
since we just changed the trigger to "pull_request_target" the variable
no longer contains the PR number and is therefore not unique and can't be
used as a concurrency group id.  We now use
`github.triggering_actor-github.head_ref`.

(cherry picked from commit 32b0b47348)

.github/workflows/OnPRCPCheck.yml

@@ -9,6 +9,6 @@ jobs:
     name: "run-cpcheck"
     if: ${{ github.event.label.name == vars.CHERRY_PICK_TEST_LABEL }}
     concurrency:
-      group: cpcheck-${{ github.base_ref }}-${{ github.ref_name }}
+      group: cpcheck-${{ github.triggering_actor }}-${{ github.head_ref }}
       cancel-in-progress: true
     uses: asterisk/asterisk-ci-actions/.github/workflows/AsteriskPRCPCheck.yml@main