.github/workflows/release: make sure GH_TOKEN is set for upload

Author: croissanne

.github/workflows/release.yml

@@ -39,6 +39,8 @@ jobs:
       # Source0 expands to `https://github.com/osbuild/image-builder-frontend/releases/download/v$VERSION/cockpit-image-builder-v$VERSION.tar.gz`,
       # so the v needs to be in the tarball when we upload it as a release artefact.
       - name: Upload release artefact
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
           gh release upload ${{github.ref_name}} \
             ../cockpit-image-builder-${{github.ref_name}}.tar.gz