ci: add concurrency guard to E2E workflow

Prevents parallel E2E runs from conflicting over the shared
headlamp-e2e Helm release in privilegedescalation-dev. With
cancel-in-progress: true, a new push cancels any in-progress
run on the same repo — only one E2E suite runs at a time.

Observed failure: PR#109 and PR#108 ran concurrently and the
auth setup in PR#109 timed out, likely due to resource contention
on the shared headlamp-e2e instance.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/e2e.yaml

@@ -10,6 +10,12 @@ on:
 permissions:
   contents: read
 
+# Only one E2E run at a time: the shared E2E_RELEASE (headlamp-e2e) in
+# privilegedescalation-dev cannot be shared across concurrent runs.
+concurrency:
+  group: e2e-${{ github.repository }}
+  cancel-in-progress: true
+
 env:
   E2E_NAMESPACE: privilegedescalation-dev
   E2E_RELEASE: headlamp-e2e