Change _BlockWinMDsOnUnsupportedTFMs dependencies to fix WPF temp targets ordering (#12104)

The introduction of _BlockWinMDsOnUnsupportedTFMs broke the WPF temp project generation, which broke PowerShell Core. This PR changes the target ordering so it doesn't destabilize the WPF targets.

I've validated that this change doesn't cause this target to be skipped on incremental builds.

Fixes #12093

cc: @rlander @AaronRobinsonMSFT @ryalanms @wli3 @dsplaisted

Author: jkoritzinsky

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.targets

@@ -891,7 +891,7 @@ Copyright (c) .NET Foundation. All rights reserved.
   ============================================================
   -->
   <Target Name="_BlockWinMDsOnUnsupportedTFMs"
-          BeforeTargets="CoreCompile"
+          AfterTargets="PreBuildEvent"
           DependsOnTargets="ResolveReferences"
           Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp' and $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '5.0'))">
     <NETSdkError Condition="'%(ReferencePath.Extension)' == '.winmd'"