Update submodules in automated maintenance action

Author: wata727

.github/workflows/maintenance.yml

@@ -15,6 +15,7 @@ jobs:
           go-version-file: 'go.mod'
       - name: make tflint in tools/magic-modules
         run: |
+          git submodule update --remote
           go install golang.org/x/tools/cmd/goimports@latest
           cd tools/magic-modules
           make tflint