ci: improve caching for GitHub Actions

Previously the cache would be stale for every new branch.
With this change, PRs use the cache from the base branch and therefore
don't need to rebuild LLVM from scratch.

.github/workflows/windows.yml

@@ -1,6 +1,11 @@
 name: Windows
 
-on: push
+on:
+  pull_request:
+  push:
+    branches:
+      - dev
+      - release
 
 jobs:
   build-windows: