Make slack channel id a repo Actions variable

Author: BotReleaser

.github/workflows/post-release.yml

@@ -109,7 +109,7 @@ jobs:
           # Slack channel id, channel name, or user id to post message.
           # See also: https://api.slack.com/methods/chat.postMessage#channels
           # On special mentions: https://api.slack.com/reference/surfaces/formatting#special-mentions
-          channel-id: 'C02Q2772VEH'
+          channel-id: ${{ vars.SLACK_CHANNEL_ID }}
           slack-message: "<!channel> New release: ${{  github.ref_name }} is being made !:tada:\nRelease notes there: ${{  steps.github-release.outputs.url }}"
           # For posting a rich message using Block Kit
         env: