Set an explicit PR reviewer for CNB builder release PRs (#1053)

Currently for CNB releases, the PR opened against `cnb-builder-images`
doesn't have an explicit reviewer set by the automation, which means it
uses that repo's `CODEOWNERS` default of requesting review from the
whole Languages team.

As of heroku/languages-github-actions#289, the
automation now supports passing a list of reviewers, which we can set
for CNBs owned by a specific language owner.

This will help reduce review-request-spam to other team members.

GUS-W-18011095.

Author: edmorley

.github/workflows/release.yml

@@ -15,6 +15,7 @@ jobs:
     with:
       app_id: ${{ vars.LINGUIST_GH_APP_ID }}
       dry_run: ${{ inputs.dry_run }}
+      reviewers: 'colincasey'
     secrets:
       app_private_key: ${{ secrets.LINGUIST_GH_PRIVATE_KEY }}
       cnb_registry_token: ${{ secrets.CNB_REGISTRY_RELEASE_BOT_GITHUB_TOKEN }}