Fix condition for setting if symbol is a file in native AOT publish targets (dotnet#90438)

Author: elinor-fung

src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Publish.targets

@@ -104,7 +104,7 @@
       <_symbolSourcePath>$(NativeOutputPath)$(TargetName)$(_symbolExt)</_symbolSourcePath>
       <_symbolTargetPath>$(PublishDir)\$(TargetName)$(_symbolExt)</_symbolTargetPath>
       <!-- If the symbol is a dsym bundle, it's a folder not a file. -->
-      <_symbolIsFile Condition="'$(_symbolExt)' == '.dsym'">false</_symbolIsFile>
+      <_symbolIsFile Condition="'$(NativeSymbolExt)' == '.dsym'">false</_symbolIsFile>
       <_symbolIsFile Condition="'$(_symbolIsFile)' == ''">true</_symbolIsFile>
     </PropertyGroup>