workflows/pr: block merging PRs when jobs have been cancelled

This currently happens, for still unknown reasons, for the "check cherry
picks" job. The job gets cancelled by GHA mid-way. This should be the
same as an error, because an important check didn't run: Merging should
be blocked and auto-merge should not succeed.

(cherry picked from commit 06f574addf)

.github/workflows/pr.yml

@@ -160,7 +160,7 @@ jobs:
     # Do NOT change the name of this job, otherwise the rule will not catch it anymore.
     # This would prevent all PRs from merging.
     name: no PR failures
-    if: ${{ failure() }}
+    if: ${{ cancelled() || failure() }}
     runs-on: ubuntu-24.04-arm
     steps:
       - run: exit 1