ci: Notify slack when miri checks fail (#1614)

Using the new notify-slack workflow from
CQCL/hugrverse-actions#18.
I'll update the target branch once that is merged.

.github/workflows/unsoundness.yml

@@ -21,9 +21,10 @@ env:
 
 jobs:
 
-   miri:
+  miri:
     name: "Miri"
     runs-on: ubuntu-latest
+    continue-on-error: true
     steps:
       - uses: actions/checkout@v4
       - name: Install Miri
@@ -38,3 +39,20 @@ jobs:
         run: sudo apt-get install -y capnproto
       - name: Test with Miri
         run: cargo miri test
+
+
+  notify-slack:
+    uses: CQCL/hugrverse-actions/.github/workflows/slack-notifier.yml@main
+    needs: miri
+    if: ${{ needs.miri.result == 'failure' && ( github.event_name == 'workflow_dispatch' || github.event_name == 'push' ) }}
+    with:
+      channel-id: 'C04SHCL4FKP'
+      slack-message: |
+        💥 The unsoundness check for `CQCL/hugr` failed. <https://github.com/CQCL/hugr/actions/runs/${{ github.run_id }}|Please investigate>.
+      # Rate-limit the message to once per day
+      timeout-minutes: 1440
+      # A repository variable used to store the last message timestamp.
+      timeout-variable: "UNSOUNDNESS_MSG_SENT"
+    secrets:
+        SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
+        GITHUB_PAT: ${{ secrets.HUGRBOT_PAT }}