check that go.mod is tidied

.github/workflows/lint.yml

@@ -4,8 +4,16 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - run: go install ./...
       - name: Check that no non-test files import Ginkgo or Gomega
         run: .github/workflows/no_ginkgo.sh
+      - name: Check that go.mod is tidied
+        run: |
+          cp go.mod go.mod.orig
+          cp go.sum go.sum.orig
+          go mod tidy
+          cmp go.mod go.mod.orig
+          cmp go.sum go.sum.orig
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v2
         with: