Enforce gofmt in GitHub Actions

Author: tianon

.github/workflows/ci.yml

@@ -36,5 +36,7 @@ jobs:
           name: coverage
           path: .test/coverage**
           if-no-files-found: error
+      - name: gofmt
+        run: find -name '*.go' -type f -exec ./.go-env.sh gofmt -l -s -w '{}' +
       - run: git diff --exit-code
       # TODO download latest coverage artifacts from HEAD / PR target to emulate Codecov but without another flaky third-party service that's begging for write-access to all our repositories via a GitHub App? 👀