ci: add temporary actions/cache smoke test

Verifying actions/cache is functional on this Gitea Actions runner
before building CI speed improvements on top of it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/_cache-smoke-test.yml

@@ -0,0 +1,40 @@
+name: Cache smoke test
+
+on:
+  push:
+    branches: [ci-cache-speedup]
+
+jobs:
+  cache-write:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Create cache payload
+        run: |
+          mkdir -p /tmp/cache-smoke
+          echo "hello-${{ github.run_id }}" > /tmp/cache-smoke/data.txt
+
+      - uses: actions/cache@v4
+        with:
+          path: /tmp/cache-smoke
+          key: cache-smoke-${{ github.run_id }}
+
+  cache-read:
+    runs-on: ubuntu-latest
+    needs: cache-write
+    steps:
+      - uses: actions/cache@v4
+        id: restore
+        with:
+          path: /tmp/cache-smoke
+          key: cache-smoke-${{ github.run_id }}
+
+      - name: Verify cache was restored
+        run: |
+          if [ "${{ steps.restore.outputs.cache-hit }}" != "true" ]; then
+            echo "cache-hit was not true: '${{ steps.restore.outputs.cache-hit }}'" >&2
+            exit 1
+          fi
+          test -f /tmp/cache-smoke/data.txt
+          cat /tmp/cache-smoke/data.txt
+          test "$(cat /tmp/cache-smoke/data.txt)" = "hello-${{ github.run_id }}"
+          echo "actions/cache works on this runner"