Fix macos_platform_tests package build times out when built for all p…

…ackages (flutter#6463)

Fixes flutter#146254

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packages/in_app_purchase/in_app_purchase_storekit/example/macos/Runner.xcodeproj/xcshareddata/xcschemes/Runner.xcscheme

@@ -39,7 +39,7 @@
       <Testables>
          <TestableReference
             skipped = "NO"
-            parallelizable = "YES">
+            parallelizable = "NO">
             <BuildableReference
                BuildableIdentifier = "primary"
                BlueprintIdentifier = "F700DD0128E652A10004836B"