chore(ci): fix concurrency group format on pull request event

Since the addition of pull_request_target event, github.ref context
object return name of the base branch. So when a workflow was
triggered on the base branch during an execution in a pull request,
the latter would be cancelled.
Using github.head_ref, when available, fixes this behavior.
On any other event than pull_request or pull_request_target,
github.ref will still be used and work as before.

.github/workflows/aws_tfhe_fast_tests.yml

@@ -159,7 +159,7 @@ jobs:
       (github.event_name == 'pull_request' && needs.setup-instance.result != 'skipped')
     needs: [ should-run, setup-instance ]
     concurrency:
-      group: ${{ github.workflow }}_${{ github.ref }}
+      group: ${{ github.workflow }}_${{ github.head_ref || github.ref }}
       cancel-in-progress: true
     runs-on: ${{ needs.setup-instance.outputs.runner-name }}
     steps: