[CQ] analyze triage source in presubmit (#8285)

Following up from a conversation w/ @jwren. Enables analysis on `triage`
tools as part of our presubmit.

---

- [x] I’ve reviewed the contributor guide and applied the relevant
portions to this PR.

<details>
  <summary>Contribution guidelines:</summary><br>

- See our [contributor
guide]([https://github.com/dart-lang/sdk/blob/main/CONTRIBUTING.md](https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview)
for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before
creating a PR.
- Dart contributions to our repos should follow the [Dart style
guide](https://dart.dev/guides/language/effective-dart) and use `dart
format`.
- Java and Kotlin contributions should strive to follow Java and Kotlin
best practices
([discussion](#8098)).
</details>

Author: pq

tool/github.sh

@@ -47,6 +47,7 @@ if [ "DART_BOT" = "$BOT" ] ; then
   echo "dart analyze"
   (cd flutter-idea/src; dart analyze)
   (cd tool/plugin; dart analyze)
+  (cd tool/triage; dart pub upgrade && dart analyze)
 
   # Ensure that the edits have been applied to template files (and their target
   # files have been regenerated).