This is a possible fix for the 'Waiting for the other yarn instance to finish' error that appears during our CI builds This property is configured in [CSharp.Common.props](https://github.com/dotnet/aspnetcore/blob/master/eng/targets/CSharp.Common.props#L57-L61) but not the NPM one. However it is used in exactly one place in the [targets](https://github.com/dotnet/aspnetcore/blob/master/eng/targets/Npm.Common.targets#L45) This change updates the target to use it more consistently and to declare it it in the props file
