chore(ci): display head ref in slack notification message

Default 'Ref' displayed in message relies on github.ref value. On
pull_request_target, it's the base_ref, instead of head_ref, that
is set as value for github.ref.
We cannot change 'Ref' field directly. As a workaround, we hide
'Ref' in the message and display the head_ref directly in
SLACK_MESSAGE.

.github/workflows/aws_tfhe_fast_tests.yml

@@ -11,6 +11,8 @@ env:
   SLACK_ICON: https://pbs.twimg.com/profile_images/1274014582265298945/OjBKP9kn_400x400.png
   SLACK_USERNAME: ${{ secrets.BOT_USERNAME }}
   SLACK_WEBHOOK: ${{ secrets.SLACK_WEBHOOK }}
+  MSG_MINIMAL: event,action url,commit
+  BRANCH: ${{ github.head_ref || github.ref }}
   IS_PULL_REQUEST: ${{ github.event_name == 'pull_request' || github.event_name == 'pull_request_target' }}
   REF: ${{ github.event.pull_request.head.sha || github.sha }}
 
@@ -287,7 +289,7 @@ jobs:
         uses: rtCamp/action-slack-notify@c33737706dea87cd7784c687dadc9adf1be59990
         env:
           SLACK_COLOR: ${{ job.status }}
-          SLACK_MESSAGE: "Fast AWS tests finished with status: ${{ job.status }}. (${{ env.ACTION_RUN_URL }})"
+          SLACK_MESSAGE: "Fast AWS tests finished with status: ${{ job.status }} on '${{ env.BRANCH }}'. (${{ env.ACTION_RUN_URL }})"
 
   teardown-instance:
     name: Teardown instance (fast-tests)
@@ -311,4 +313,4 @@ jobs:
         uses: rtCamp/action-slack-notify@c33737706dea87cd7784c687dadc9adf1be59990
         env:
           SLACK_COLOR: ${{ job.status }}
-          SLACK_MESSAGE: "Instance teardown (fast-tests) finished with status: ${{ job.status }}. (${{ env.ACTION_RUN_URL }})"
+          SLACK_MESSAGE: "Instance teardown (fast-tests) finished with status: ${{ job.status }} on '${{ env.BRANCH }}'. (${{ env.ACTION_RUN_URL }})"