Avoid using TargetFramework in build targets (#36545)

Fixes #36510

Co-authored-by: Pranav K <[email protected]>

Author: github-actions[bot]

src/Mvc/Mvc.Testing/src/buildTransitive/Microsoft.AspNetCore.Mvc.Testing.targets

@@ -1,5 +1,5 @@
 <Project>
 
-  <Import Project="..\..\build\$(TargetFramework)\Microsoft.AspNetCore.Mvc.Testing.targets" />
+  <Import Project="..\..\build\net6.0\Microsoft.AspNetCore.Mvc.Testing.targets" />
 
 </Project>