add slack notification to release PPA

.github/workflows/post-publish.yml

@@ -26,3 +26,15 @@ jobs:
           token: ${{ secrets.CRYPTOBOT_RELEASE_TOKEN }}
           files: |
             cryptomator-*.tar.gz.asc
+      - name: Slack Notification
+        uses: rtCamp/action-slack-notify@v2
+        env:
+          SLACK_WEBHOOK: ${{ secrets.SLACK_WEBHOOK_URL }}
+          SLACK_USERNAME: 'Cryptobot'
+          SLACK_ICON: false
+          SLACK_ICON_EMOJI: ':bot:'
+          SLACK_CHANNEL: 'cryptomator-desktop'
+          SLACK_TITLE: "Release ${{ github.event.repository.name }} ${{ github.event.release.tag_name }} published."
+          SLACK_MESSAGE: "Ready to <https://github.com/${{ github.repository }}/actions/workflows/debian.yml|deploy to PPA>."
+          SLACK_FOOTER: false
+          MSG_MINIMAL: true