ci: run pull_request checks for PRs based on any branch

'*' does not match '/' in GitHub branch filters, so a PR whose base is a
feature branch (e.g. a stacked PR) never triggered CI.

.github/workflows/ci.yaml

@@ -3,7 +3,7 @@ name: CI
 on:
   pull_request:
     branches:
-    - '*'
+    - '**'
   push:
     branches:
     - main