build: fix GitHub API rate limit errors when fetching GUI dist in CI

The fetch-gui-dist.sh script calls the GitHub releases API
unauthenticated, which is limited to 60 requests/hour per source IP.
GitHub Actions runners share outbound IPs, so this quota is regularly
exhausted.

Pass GITHUB_TOKEN (or GH_TOKEN) as an Authorization header when
present, raising the limit to 1000/hour, and wire secrets.GITHUB_TOKEN
into the workflow step. Local unauthenticated runs still work.

.github/workflows/build.yml

@@ -233,6 +233,8 @@ jobs:
           
       - name: Fetch GUI dist
         run: make fetch-gui
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Cache
         uses: actions/cache@v5