Fix slnx restore (#15897)

Author: ViktorHofer

src/Microsoft.DotNet.Arcade.Sdk/tools/Build.proj

@@ -221,7 +221,7 @@
              Targets="_IsProjectRestoreSupported"
              SkipNonexistentTargets="true"
              BuildInParallel="true"
-             Condition="'$(RestoreUsingNuGetTargets)' != 'false' and '%(ProjectToBuild.Extension)' != '.sln' and '%(ProjectToBuild.Extension)' != '.slnf' and '$(RestoreUseStaticGraphEvaluation)' != 'true' and '$(Restore)' == 'true'">
+             Condition="'$(RestoreUsingNuGetTargets)' != 'false' and '%(ProjectToBuild.Extension)' != '.sln' and '%(ProjectToBuild.Extension)' != '.slnf' and '%(ProjectToBuild.Extension)' != '.slnx' and '$(RestoreUseStaticGraphEvaluation)' != 'true' and '$(Restore)' == 'true'">
 
       <Output TaskParameter="TargetOutputs" ItemName="_ProjectToRestoreWithNuGet" />
     </MSBuild>