ci: read the app token from the release environment

RELEASE_APP_ID and RELEASE_APP_PRIVATE_KEY already live in the release
environment of mercure and frankenphp, so reuse it rather than copying
the private key into a second environment.

Author: dunglas

.github/workflows/dependabot.yaml

@@ -10,7 +10,7 @@ permissions: {}
 jobs:
   auto-merge:
     runs-on: ubuntu-latest
-    environment: dependabot
+    environment: release
     if: github.event.pull_request.user.login == 'dependabot[bot]'
     steps:
       # The default GITHUB_TOKEN lacks the `workflows` scope and is refused on