Fix update workflow

There seem to be a bug in Git 2.54.0 leading to "fatal: shallow file has changed since we read it" errors.

I have confirmed this fix in a test repo.

Fixes #773

.github/workflows/update-themes.yml

@@ -20,6 +20,9 @@ jobs:
       - uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
         with:
           go-version: "^1.27.0"
+        # Bug in Git 2.54.0 leading to "fatal: shallow file has changed since we read it" errors.
+      - name: Disable Git Auto-Maintenance
+        run: git config --global maintenance.auto false
       - name: Update submodules
         working-directory: ./cmd/hugothemesitebuilder
         run: |