gha: add ci job to check go mod and vendor

In order to avoid forgetting to clean this up.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

.github/workflows/go.yml

@@ -21,6 +21,15 @@ jobs:
       with:
         go-version: ${{ matrix.go }}
 
+    - name: Go Modules
+      run: |
+        go mod tidy
+        git diff --exit-code
+
+        go mod vendor
+        git add -N vendor/
+        git diff --exit-code
+
     - name: Build
       run: make build