Extend CI workflow with gofmt

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

.github/workflows/ci.yml

@@ -12,6 +12,8 @@ jobs:
       uses: actions/setup-go@v1
       with:
         go-version: 1.14
+    - name: Ensure gofmt
+      run: test -z "$(gofmt -s -d .)"
     - name: Ensure go.mod is already tidied
       run: go mod tidy && git diff --no-patch --exit-code
     - name: Run unit tests