github-ci: add workflow with checking format

Author: ligurio

.github/workflows/check.yaml

@@ -0,0 +1,18 @@
+name: Run checks
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  run-format:
+    if: github.event_name == 'push' ||
+      github.event.pull_request.head.repo.full_name != github.repository
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@master
+
+    - name: Check format
+      run: if [ "$(gofmt -s -l . | wc -l)" -gt 0 ]; then exit 1; fi