Add rubocop cache restore key by github.ref

Author: lovro-bikic

.github/workflows/build.yml

@@ -75,8 +75,9 @@ jobs:
         uses: actions/cache@v2
         with:
           path: tmp/rubocop-cache
-          key: ${{ runner.os }}-rubocop-cache-${{ github.sha }}
+          key: ${{ runner.os }}-rubocop-cache-${{ github.ref }}-${{ github.sha }}
           restore-keys: |
+            ${{ runner.os }}-rubocop-cache-${{ github.ref }}-
             ${{ runner.os }}-rubocop-cache-
       - name: Set up Node
         uses: actions/setup-node@v2