Added import of .user file for multi-targeted builds (dotnet#9444)

Fixes dotnet#9131

Context
As described on the issue, muti-targeted builds did not import the .user file on the outer build. This change makes the outer build import the .user file.

Changes Made
Added import reference to .user file in  Microsoft.Common.CrossTargeting.targets .

Testing
Test is in SDK repo (dotnet/sdk#37192)

src/Tasks/Microsoft.Common.CrossTargeting.targets

@@ -19,6 +19,8 @@ Copyright (C) Microsoft Corporation. All rights reserved.
   <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.CrossTargeting.targets\ImportBefore\*.targets"
           Condition="'$(ImportByWildcardBeforeMicrosoftCommonCrossTargetingTargets)' == 'true' and exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.CrossTargeting.targets\ImportBefore')"/>
 
+  <Import Project="$(MSBuildProjectFullPath).user" Condition="Exists('$(MSBuildProjectFullPath).user')"/>
+
   <Import Project="$(CustomBeforeMicrosoftCommonCrossTargetingTargets)" Condition="'$(CustomBeforeMicrosoftCommonCrossTargetingTargets)' != '' and Exists('$(CustomBeforeMicrosoftCommonCrossTargetingTargets)')"/>
 
   <Target Name="GetTargetFrameworks"