Fix target frameworks for Source Build (#5753)

Author: jeffkl

build/common.project.props

@@ -19,6 +19,7 @@
 
     <!-- Target frameworks for class libraries-->
     <TargetFrameworksLibrary>$(NetStandardVersion)</TargetFrameworksLibrary>
+    <TargetFrameworksLibrary Condition="'$(DotNetBuildFromSource)' == 'true'">$(NETCoreTargetFramework);$(TargetFrameworksLibrary)</TargetFrameworksLibrary>
     <TargetFrameworksLibrary Condition="'$(IsXPlat)' != 'true' And '$(DotNetBuildFromSource)' != 'true'">$(NETFXTargetFramework);$(TargetFrameworksLibrary)</TargetFrameworksLibrary>
 
     <!-- Target frameworks for class libraries which require signing APIs which need to target NET 5.0 -->