ci: Pass GITHUB_TOKEN to release-plz

The action needs the token to call the GitHub API (open release PR,
create tags, create releases). Workflows have to forward it
explicitly; it is not auto-injected as an env var even though the
secret is auto-provisioned per run.

Author: RReverser

.github/workflows/release.yml

@@ -21,3 +21,5 @@ jobs:
       - uses: release-plz/action@v0.5
         with:
           verbose: true
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}