Explicitly set GITHUB_BASE_URL for modulesync

This should fix the modulesync errors that are popping up for recent
runs, e.g. https://github.com/projectsyn/modulesync-control/runs/5630431030?check_suite_focus=true

See the modulesync issue at voxpupuli/modulesync#250

.github/workflows/sync.yml

@@ -33,6 +33,7 @@ jobs:
       - name: Run modulesync
         env:
           GITHUB_TOKEN: ${{ secrets.MODULESYNC_TOKEN }}
+          GITHUB_BASE_URL: "https://api.github.com"
         run: |
           sha="$(git rev-parse --short HEAD)"
           msync_args="--filter=${{ github.event.inputs.filter }}"