SourceLink fails build with MSBuild 15.1

[AArnott]

On a machine with VS2017 RTW, and in a VS 2017 Developer Command Prompt

git clone https://github.com/aarnott/pinvoke
cd pinvoke
git checkout EnableSourceLink
cd src\Kernel32
msbuild /v:minimal /nologo /nr:false /t:restore
msbuild /v:minimal /nologo /m /nr:false /t:pack /p:ci=true

C:\Users\andrew.nuget\packages\sourcelink.create.github\2.0.0\build\SourceLink.Create.GitHub.targets(21,5): error MSB4062: The "SourceLink.Create.GitHub.CreateTask" task could not be loaded from the assembly C:\Users\andrew.nuget\packages\sourcelink.create.github\2.0.0
build\SourceLink.Create.GitHub.dll. Could not load file or assembly 'System.Runtime, Version=4.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified. Confirm that the declaration is correct, that the assembly and all its dependencies are available, and that the task contains a public class that implements Microsoft.Build.
Framework.ITask. [G:\git\pinvoke\src\CodeGenerationAttributes\CodeGenerationAttributes.csproj]

[AArnott]

Dupe of #156

[ctaggart]

Looks like a great test for #164.

[ctaggart]

Closing this in favor of #164.