fix(windows): fix CppWinRTIncludes.h not being found (#2388)

Depending on how `react-native-windows` was hoisted, build may fail to
find the header file.

Author: tido64

windows/Win32/ReactApp.vcxproj

@@ -19,7 +19,7 @@
     <ReactAppWin32Dir Condition="'$(ReactAppWin32Dir)'==''">$(ReactAppWinDir)\Win32</ReactAppWin32Dir>
     <ReactAppGeneratedDir Condition="'$(ReactAppGeneratedDir)'==''">$(MSBuildProjectDirectory)\..\..</ReactAppGeneratedDir>
     <ReactNativeDir Condition="$(UseExperimentalNuget)=='false' AND '$(ReactNativeDir)'==''">$([MSBuild]::GetDirectoryNameOfFileAbove($(SolutionDir), 'node_modules\react-native\package.json'))\node_modules\react-native\</ReactNativeDir>
-    <ReactNativeWindowsDir Condition="'$(ReactNativeWindowsDir)'==''">$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), 'node_modules\react-native-windows\package.json'))\node_modules\react-native-windows\</ReactNativeWindowsDir>
+    <ReactNativeWindowsDir Condition="'$(ReactNativeWindowsDir)'==''">$([MSBuild]::GetDirectoryNameOfFileAbove($(SolutionDir), 'node_modules\react-native-windows\package.json'))\node_modules\react-native-windows\</ReactNativeWindowsDir>
   </PropertyGroup>
   <Import Project="$(ReactNativeWindowsDir)\PropertySheets\External\Microsoft.ReactNative.WindowsSdk.Default.props" />
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />