CI: Add GH_TOKEN to publish workflow

When trying to release a minor version, GitHub Actions said:

    gh: To use GitHub CLI in a GitHub Actions workflow, set the GH_TOKEN
    environment variable. Example:
      env:
        GH_TOKEN: ${{ github.token }}

we didn't notice it before, because this code doesn't run on patch
releases.

.github/workflows/publish.yml

@@ -28,6 +28,8 @@ jobs:
             --title "RuboCop RSpec $(git tag --points-at @)" \
             --notes-file relnotes.md
       - name: Replace version in Antora config
+        env:
+          GH_TOKEN: ${{ github.token }}
         run: |
           sed -i 's/version:.*$/version: ~/' docs/antora.yml
           if ! git diff --exit-code docs/antora.yml; then