[CI] Github issues & external PRs fix (MystenLabs#17616)

## Description 

If conditions only work at job level, and not at step level.

## Test plan 

How did you test the new or updated feature?

---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:

.github/workflows/github-issues-monitor.yml

@@ -7,20 +7,24 @@ on:
     types: [opened]
 
 jobs: 
-  notify:
-    name: Dispatch workflow to notify slack channel
+  notify-prs:
+    name: Dispatch workflow to notify slack channel on PRs
+    if: github.event_name == 'pull_request'
     runs-on: ubuntu-latest
     steps:
       - name: Dispatch github-issues-external-prs-monitor in MystenLabs/sui-operations
-        if: ${{github.event_name == 'pull_request'}}
         uses: peter-evans/repository-dispatch@ff45666b9427631e3450c54a1bcbee4d9ff4d7c0 # pin@v3.0.0
         with: 
           repository: MystenLabs/sui-operations
           event-type: github-issues-external-prs-monitor
           client-payload: '{"author": "${{github.event.pull_request.user.login}}", "event_name": "pull_request", "pull_request_number": "${{github.event.pull_request.number}}"}'
 
+  notify-issues:
+    name: Dispatch workflow to notify slack channel on issues
+    if: github.event_name == 'issues'
+    runs-on: ubuntu-latest
+    steps:
       - name: Dispatch github-issues-external-prs-monitor in MystenLabs/sui-operations
-        if: ${{github.event_name == 'issues'}}
         uses: peter-evans/repository-dispatch@ff45666b9427631e3450c54a1bcbee4d9ff4d7c0 # pin@v3.0.0
         with: 
           repository: MystenLabs/sui-operations