Fix CoreLib to include PasteArguments.Unix.cs correctly (#22453)

Author: stephentoub

src/System.Private.CoreLib/shared/System.Private.CoreLib.Shared.projitems

@@ -1159,7 +1159,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\PathInternal.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\PersistedFiles.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\PersistedFiles.Names.Unix.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\PasteArguments.Windows.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\PasteArguments.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\MemoryFailPoint.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\Marshal.Unix.cs" Condition="'$(TargetsCoreRT)' != 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Security\SecureString.Unix.cs" />