ci: use head_ref for concurrency group

Use the head_ref value for the concurrency group on `ci-privileged` rather than the ref, so that the main ref is not used and cancelling across pull requests and the main branch

Backport angular/angular#51784 to the CLI.

(cherry picked from commit e6e6d0c)

Author: alan-agius4

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ on:
     types: [opened, synchronize, reopened]
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.head_ref }}
   cancel-in-progress: true
 
 permissions: {}