ci(github): attempt to speed up update workflow with cache

.github/workflows/update-formulas.yml

@@ -42,7 +42,14 @@ jobs:
       - uses: actions/setup-go@v2
         with:
           go-version: 1.16
-      - name: Pre-build emacs-builder tool
+      - uses: actions/cache@v2
+        id: builder-cache
+        with:
+          path: ~/go/pkg/mod
+          key: ${{ runner.os }}-go-${{ hashFiles('builder/**/go.sum') }}
+          restore-keys: ${{ runner.os }}-go-
+      - name: Build emacs-builder tool
+        if: steps.cache.outputs.cache-hit != 'true'
         run: make build
         working-directory: builder
       - name: Update Cask formulas