fix(windows): do not set ReactNativeDir unless building from source

[tido64]

Description

Nightlies are failing because of ReactNativeDir being set. I'm not entirely sure why, but unsetting it makes the errors go away:

- Build failed with message D:\a\react-native-test-app\react-native-test-app\node_modules\react-native\ReactCommon\react\nativemodule\core\ReactCommon\TurboModule.cpp(9,10): error C1083: Cannot open include file: 'react/debug/react_native_assert.h': No such file or directory [D:\a\react-native-test-app\react-native-test-app\example\node_modules\.generated\windows\UWP\ReactTestApp.vcxproj]. Check your build configuration.

-- https://github.com/microsoft/react-native-test-app/actions/runs/9591391859/job/26448224357

Platforms affected

• Android
• iOS
• macOS
• visionOS
• Windows

Test plan

npm run set-react-version canary-windows
yarn clean
yarn
cd example
yarn install-windows-test-app --use-nuget
yarn windows

[tido64]

FYI @Yajur-Grover