Make the build more robust

Most notably:

* Make sure that conig.json is recreated on every build.
* Also create that file as the first part of the "version updates"

This should allow for both deletions/mods in both themes.txt and go.mod to work around missing refs etc.

If this isn't good enough, we could consider just adding the go.mod to .gitignore.

.github/workflows/update-themes.yml

@@ -17,8 +17,10 @@ jobs:
           go-version: "^1.21.0"
       - name: Update submodules
         shell: bash
-        working-directory: ./cmd/hugothemesitebuilder/build
+        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 }}