ci: add workflow to cleanup per-PR Actions caches on close

.github/workflows/cleanup-pr-caches.yml

@@ -0,0 +1,37 @@
+name: Cleanup caches for closed PRs
+
+on:
+  pull_request:
+    types:
+      - closed
+
+permissions:
+  actions: write
+  contents: read
+
+jobs:
+  cleanup:
+    name: Delete caches for the closed PR
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/github-script@v7
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            const ref = `refs/pull/${context.payload.pull_request.number}/merge`;
+            console.log(`Listing caches for ref: ${ref}`);
+            const { data } = await github.rest.actions.getActionsCacheList({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              ref,
+            });
+            console.log(`Found ${data.actions_caches.length} cache(s) to delete.`);
+            for (const cache of data.actions_caches) {
+              console.log(`Deleting cache id=${cache.id} key=${cache.key}`);
+              await github.rest.actions.deleteActionsCacheById({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                cache_id: cache.id,
+              });
+            }
+            console.log('Cleanup finished.');