[mono] Fix VSMac build with .netcore 2.1 installed (#61) (#3384)
[mono] Fix VSMac build with .netcore 2.1 installed (#61) (#3384)

For the mono build we have a dependency on
`Microsoft.Extensions.DependencyModel`, and so we get the assembly in
the binDir.

When building a netstandard project, the MSBuild sdk resolver resolves
to the installed dotnet (2.1 RC in this case).
`Microsoft.NET.Build.Tasks` is loaded from there but it's dependency is
available in msbuild's(entry assembly) binDir itself, so it gets loaded
from there.  And because the one from dotnet has a new
class `Microsoft.Extensions.DependencyModel.RuntimeFile`,
this fails at runtime with:

`System.TypeLoadException: Could not resolve type with token 01000030 from typeref (expected class 'Microsoft.Extensions.DependencyModel.RuntimeFile' in assembly 'Microsoft.Extensions.DependencyModel, Version=2.1.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60')`

This assembly isn't required for mono msbuild because it is meant for
reading `.deps` files. And this does not affect building netcoreapps,
because in that case the sdk resolver uses the installed `dotnet`. And
if it isn't installed, then we just use our bundled SDK which already
has this assembly.

Fixes bug: https://github.com/mono/mono/issues/8715
Microsoft/msbuild,rainersigwald/msbuild,mono/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,cdmihai/msbuild,Microsoft/msbuild,Microsoft/msbuild,mono/msbuild,sean-gilliam/msbuild,chipitsine/msbuild,chipitsine/msbuild,rainersigwald/msbuild,jeffkl/msbuild,sean-gilliam/msbuild,mono/msbuild,chipitsine/msbuild,cdmihai/msbuild,rainersigwald/msbuild,Microsoft/msbuild,mono/msbuild,jeffkl/msbuild,sean-gilliam/msbuild,jeffkl/msbuild,cdmihai/msbuild,rainersigwald/msbuild,chipitsine/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,AndyGerlicher/msbuild,AndyGerlicher/msbuild