[infra] upload build report artifacts (#8763)

Uploads build artifacts (test and verifier reports) after all actions.

Artifact URLs can be found in the run output:

<img width="916" height="425" alt="image"
src="https://github.com/user-attachments/assets/fd626ab2-a804-4f47-9b35-a6c94a9d322d"
/>

---

- [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/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md](https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md)
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

.github/workflows/presubmit.yaml

@@ -36,3 +36,11 @@ jobs:
         run: ./tool/github.sh
         env:
           BOT: ${{ matrix.bot }}
+      - name: upload build artifacts
+        uses: actions/upload-artifact@v4
+        if: always()
+        with:
+         name: build-${{ matrix.bot }}
+         path: |
+           **/build/reports/
+           **/build/test-results/