ci: run CI on PRs and limit push event to protected branches (#751)

Run the CI on PRs, while preventing it from running twice for non-fork
PRs by limiting the push event to protected branches. [1]

This restores the workflow trigger that was accidentally modified in
commit 2b85a56 ("ci: simplify workflows").

[1]: #749 (comment)

.github/workflows/check.yml

@@ -3,6 +3,10 @@ name: Check
 
 on:  # yamllint disable-line rule:truthy
   push:
+    branches:
+      - master
+      - release-**
+  pull_request:
 
 permissions:
   contents: read