Refuse merge comment request when repository is using Release Please

[parkr]

The Jekyll repo is using the Release Please action in place of the Jekyllbot merge approach. Jekyllbot should be resilient to this by refusing a "Jekyllbot: merge" comment with a reply to new documentation if the release please workflow is in place in the repo.

[parkr]

The file to check for is ".github/workflows/release-please.yml" on the PR's base branch.