fix(ci): use head_ref in MegaLinter concurrency group to prevent self-cancellation

When a PR is merged, GitHub fires both a push event and a
pull_request[closed] event simultaneously. Since the 2023 fix to
github.ref, both events resolve to refs/heads/main, placing them in
the same concurrency group. With cancel-in-progress: true, the closed
event cancels the push run, then immediately skips (due to the
if: github.event.action != 'closed' guard), leaving no surviving
MegaLinter run.

Fix: use github.head_ref (the PR source branch) with a fallback to
github.ref (for push events). This keeps closed events in the PR's
concurrency group so they cancel in-progress PR runs as intended,
without interfering with push-triggered runs on main.

Author: ChipWolf

.github/workflows/megalinter.yml

@@ -4,18 +4,14 @@ on:
   push:
     branches: [main]
   pull_request:
-    types:
-      - opened
-      - synchronize
-      - reopened
-      - closed
+    types: [opened, synchronize, reopened, closed]
 
 permissions:
   contents: read
   security-events: write
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
   cancel-in-progress: true
 
 jobs: