ci(audience): align mobile-build triggers with playmode (SDK-329)

- Replaces mobile-build's short-form `if` with explicit enumeration of
  pull_request (non-fork), schedule, and workflow_dispatch.
- Removes reliance on `null == false` coercing to true; on schedule events
  the previous form already fired the job, but only by accident of how
  GitHub Actions normalises null to 0.
- Brings mobile-build's trigger envelope in line with the playmode job, so
  the weekly cron exercises the full mobile build matrix the same way it
  exercises the full playmode matrix.
- mobile-build matrix and steps unchanged.

Linear: https://linear.app/imtbl/issue/SDK-329

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

.github/workflows/test-audience-sample-app.yml

@@ -470,7 +470,10 @@ jobs:
   # Note: Unity 6 cells use 6000.4.5f1 (the latest patch with a GameCI image);
   # 6000.4.0f1 does not have a published GameCI Docker image.
   mobile-build:
-    if: github.event.pull_request.head.repo.fork == false || github.event_name == 'workflow_dispatch'
+    if: |
+      (github.event_name == 'pull_request' && github.event.pull_request.head.repo.fork == false)
+      || github.event_name == 'schedule'
+      || github.event_name == 'workflow_dispatch'
     name: ${{ matrix.target }} / IL2CPP / Unity ${{ matrix.unity }}
     runs-on: ubuntu-latest-8-cores
     strategy: