release-drafter: pass GITHUB_TOKEN to the action

The action reads the token from the GITHUB_TOKEN environment variable and
aborts with "token is required" when it is unset, so the workflow has been
failing on every merged PR. Set it from the built-in GITHUB_TOKEN secret, as
the hubot-kredits and kredits-web workflows already do.

.gitea/workflows/release_drafter.yml

@@ -9,3 +9,5 @@ jobs:
     steps:
       - name: Release Drafter
         uses: https://github.com/raucao/gitea-release-drafter@dev
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}