Use Dead Man's Snitch to monitor alert.yml

Use Dead Man's Snitch (the new `snitch.yml` workflow that this commit
adds) to monitor the `alert.yml` workflow and make sure it's completing
successfully, instead of having the `slack.yml` workflow post into Slack
when `alert.yml` fails.

This is better because it will alerts us if `alert.yml` is no longer
running, not only if it's running but failing.

Dead Man's Snitch will post the alerts into Slack for us.

.github/workflows/slack.yml

@@ -1,7 +1,7 @@
 name: Slack
 on:
   workflow_run:
-    workflows: [CI, Alert]
+    workflows: [CI]
     types: [completed]
     branches: [main]
 jobs:

.github/workflows/snitch.yml

@@ -0,0 +1,13 @@
+name: Dead Man's Snitch
+on:
+  workflow_run:
+    workflows: [Alert]
+    types: [completed]
+    branches: [main]
+jobs:
+  on-success:
+    runs-on: ubuntu-latest
+    if: ${{ github.event.workflow_run.conclusion == 'success' }}
+    steps:
+      - name: Check in with Dead Man's Snitch
+        run: curl https://nosnch.in/${{ secrets.SNITCH_ID }}