Skip building Apple demo app on fork PRs (#6865)

Author: huydhn

.github/workflows/apple.yml

@@ -28,6 +28,8 @@ concurrency:
 jobs:
   build-demo-ios:
     name: build-demo-ios
+    # NB: Don't run this on fork PRs because they won't have access to the secret and would fail anyway
+    if: ${{ !github.event.pull_request.head.repo.fork }}
     uses: pytorch/test-infra/.github/workflows/macos_job.yml@main
     secrets: inherit
     with:
@@ -219,6 +221,8 @@ jobs:
 
   build-benchmark-app:
     name: build-benchmark-app
+    # NB: Don't run this on fork PRs because they won't have access to the secret and would fail anyway
+    if: ${{ !github.event.pull_request.head.repo.fork }}
     uses: pytorch/test-infra/.github/workflows/macos_job.yml@main
     secrets: inherit
     with: