github: Mark the build as failed if 'do not merge' label is set

This prevents us from merging PRs marked as 'do not merge'.

Signed-off-by: Donatas Abraitis <[email protected]>

.github/workflows/freeze.yml

@@ -1,17 +1,17 @@
-name: Warn before merging if a "freeze" label exists
+name: Warn before merging if a "freeze" or "do not merge" label exists
 
 on:
   pull_request_target:
     types: [synchronize, opened, reopened, labeled, unlabeled]
 
 jobs:
   freeze_warning:
-    if: ${{ contains(github.event.*.labels.*.name, 'freeze') }}
-    name: Warn before merging if a "freeze" label exists
+    if: ${{ contains(github.event.*.labels.*.name, 'freeze') || contains(github.event.*.labels.*.name, 'do not merge') }}
+    name: Warn before merging if a "freeze" or "do not merge" label exists
     runs-on: ubuntu-latest
     steps:
       - name: Check for "freeze" label
         run: |
-          echo "Pull request is labeled as 'freeze'"
+          echo "Pull request is labeled as 'freeze' or 'do not merge'"
           echo "This workflow fails so that the pull request cannot be merged."
           exit 1