Only change platform to AnyCPU for csproj files

Directory.Build.props

@@ -6,7 +6,7 @@
     We are currently on too old of an SDK to detect - it doesn't have https://github.com/dotnet/sdk/pull/1242.
     But once we move to a new version, we can remove this property setting, and everything else will work correctly.
     -->
-    <UsingMicrosoftNETSdk>true</UsingMicrosoftNETSdk>
+    <UsingMicrosoftNETSdk Condition="'$(MSBuildProjectExtension)' == '.csproj'">true</UsingMicrosoftNETSdk>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(UsingMicrosoftNETSdk)' == 'true'">