Skip Slack review notification for Dependabot PRs

[nickclyde]

Summary

• Adds github.actor != 'dependabot[bot]' condition to the Slack review notifier workflow so it doesn't fire for automated Dependabot PRs
• Dependabot-triggered workflows don't have access to repo secrets (SLACK_WEBHOOK_URL), causing the check to fail on every Dependabot PR
• It's not worth risking leaking the webhook URL in case dependabot is compromised
• The daily open-PRs cron (slack_notifier_open_pulls.yml) already surfaces these to the team

Resolves #402

[BradySkylight]

Nice!!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.27%. Comparing base (6c63a05) to head (441c44b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #422   +/-   ##
=======================================
  Coverage   95.27%   95.27%           
=======================================
  Files          42       42           
  Lines        2161     2161           
=======================================
  Hits         2059     2059           
  Misses        102      102           

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