ci(audience): scope fork check to pull_request event so gate works on workflow_dispatch (SDK-255)

GHA expressions treat null == false as true, so on workflow_dispatch
(where github.event.pull_request is null) the first branch of the OR
gate evaluated true regardless of the selfhosted_only input. The flag
had no effect.

Wrapping the fork check in a github.event_name == 'pull_request' guard
makes that branch only run on PR events, leaving the workflow_dispatch
branch as the sole gate when the workflow is triggered manually.

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

Author: ImmutableJeffrey

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

@@ -25,7 +25,7 @@ concurrency:
 
 jobs:
   playmode:
-    if: github.event.pull_request.head.repo.fork == false || (github.event_name == 'workflow_dispatch' && github.event.inputs.selfhosted_only != 'true')
+    if: (github.event_name == 'pull_request' && github.event.pull_request.head.repo.fork == false) || (github.event_name == 'workflow_dispatch' && github.event.inputs.selfhosted_only != 'true')
     name: ${{ matrix.target }} / ${{ matrix.backend }} / Unity ${{ matrix.unity }}
     strategy:
       fail-fast: false
@@ -445,7 +445,7 @@ jobs:
   # Linux PlayMode runs on GitHub-hosted Ubuntu via GameCI Docker, not on the
   # self-hosted matrix above, to keep self-hosted machines free for Win/macOS.
   playmode-linux:
-    if: github.event.pull_request.head.repo.fork == false || (github.event_name == 'workflow_dispatch' && github.event.inputs.selfhosted_only != 'true')
+    if: (github.event_name == 'pull_request' && github.event.pull_request.head.repo.fork == false) || (github.event_name == 'workflow_dispatch' && github.event.inputs.selfhosted_only != 'true')
     name: ${{ matrix.target }} / ${{ matrix.backend }} / Unity ${{ matrix.unity }}
     runs-on: ubuntu-latest-8-cores
     strategy:
@@ -509,7 +509,7 @@ 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' && github.event.inputs.selfhosted_only != 'true')
+    if: (github.event_name == 'pull_request' && github.event.pull_request.head.repo.fork == false) || (github.event_name == 'workflow_dispatch' && github.event.inputs.selfhosted_only != 'true')
     name: ${{ matrix.target }} / IL2CPP / Unity ${{ matrix.unity }}
     runs-on: ubuntu-latest-8-cores
     strategy: