Fix ci slack notifications (only on main, not all branches) (#397)

Only notify #ci on Slack for 'main' branch.

Add token so workflow links work. From https://github.com/ravsamhq/notify-slack-action#readme:

> In order to use {workflow_url}, specify the token input as token: ${{ secrets.GITHUB_TOKEN }}.

.github/workflows/ci.yml

@@ -42,8 +42,9 @@ jobs:
       - name: Run all tests
         run: script/ci
       - uses: ravsamhq/notify-slack-action@v1
-        if: always()
+        if: github.ref_name == 'main'
         with:
+          token: ${{ secrets.GITHUB_TOKEN }}
           status: ${{ job.status }}
           notification_title: '{workflow} has {status_message}'
           message_format: '{emoji} *{workflow}* {status_message} in <{repo_url}|{repo}>'