ci: Cache Gradle on PR build

.github/workflows/build_pull_request.yml

@@ -41,6 +41,11 @@ jobs:
           channel: stable
           cache: true
           
+      - name: Cache Gradle
+        uses: burrunan/gradle-cache-action@v1
+        with:
+          build-root-directory: ${{ github.workspace }}/android
+          
       - name: Get dependencies
         run: flutter pub get