GenerateTargetFrameworkMonikerAttribute warns about access issues in parallel builds

[natemcmaster]

When build a solution in parallel, the GenerateTargetFrameworkMonikerAttribute target sometimes produces this warning:

C:\dev\dotnet\sdk\2.0.0-preview1-005418\Microsoft.Common.CurrentVersion.targets(3140,5): warning MSB3491: Could not write lines to file "C:\Users\namc\AppData\Local\Temp.NETCoreApp,Version=v1.0.AssemblyAttributes.cs". The process cannot access the file 'C:\Users\namc\AppData\Local\Temp.NETCoreApp,Version=v1.0.AssemblyAttributes.cs' because it is being used by another process. [C:\dev\DotNetTools\src\Microsoft.DotNet.Watcher.Tools\Microsoft.DotNet.Watcher.Tools.csproj]

May be resolved by #1479 or dotnet/sdk#842

cc @jeffkl

[AndyGerlicher]

Dupe of #1479