Simplify the GitHub repo cache setup and add it to the GitHub action

.github/workflows/update-themes.yml

@@ -10,6 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     env:
       HUGO_CACHEDIR: /tmp/hugo_cache
+      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-go@v3
@@ -19,11 +20,11 @@ jobs:
         shell: bash
         working-directory: ./cmd/hugothemesitebuilder
         run: |
-          go run main.go build -configOnly
-          cd build
           git config --global user.email "bep@users.noreply.github.com"
           git config --global user.name "bep"
           go install github.com/gohugoio/hugo@${{ env.HUGO_VERSION }}
+          go run main.go build -skipSiteBuild -cleanCache
+          cd build
           hugo mod get
           hugo mod tidy
           git add .