ci(audience): use github.event.inputs for selfhosted_only gate (SDK-255)

The previous gate compared inputs.selfhosted_only to the boolean true.
GitHub Actions can deliver workflow_dispatch boolean inputs as a string
('true' / 'false') depending on how the dispatch is invoked, in which
case "string != boolean" is true and the gate fails open.

github.event.inputs.X is always a string regardless of declared type, so
comparing to the string 'true' works reliably whether the input is sent
as JSON boolean, gh CLI -f (string), or gh CLI -F (raw).

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' && inputs.selfhosted_only != true)
+    if: 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' && inputs.selfhosted_only != true)
+    if: 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' && inputs.selfhosted_only != true)
+    if: 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: