chore: fix ci PRs might get merged with failing tests (#1689)

We had a PR aws/jsii-compiler#1191 that got
merged despite a failure in Node22 tests.

This happened because on a failure of one of the matrix builds, the job
that is unifying them is skipped. This is documented behavior:

![image](https://github.com/user-attachments/assets/c4eb7ae6-1b37-4f5e-91c4-65a5e5661514)

The fix is to make sure the combination step always runs and manually
check the status of the matrix build.

---

By submitting this pull request, I confirm that my contribution is made
under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

projenrc/build-workflow.ts

@@ -230,7 +230,18 @@ export class BuildWorkflow {
         needs: ['matrix-test'],
         permissions: {},
         runsOn: ['ubuntu-latest'],
-        steps: [{ name: 'Done', run: 'echo OK' }],
+        if: 'always()',
+        steps: [
+          {
+            name: 'Build result',
+            run: 'echo ${{needs.matrix-test.result}}',
+          },
+          {
+            if: "${{ needs.matrix-test.result != 'success' }}",
+            name: 'Set status based on matrix build',
+            run: 'exit 1',
+          },
+        ],
       },
       'package': {
         env: { CI: 'true' },