Allow disabling or overriding the framework path override

[AdmiringWorm]

When running a .NET Core build on Unix system we always add a framework path override.
In some cases, this may be undesirable (like if the projects have the .NET Framework assembly references).

I would suggest allowing this ability to be disabled, and optionally be allowed to pass in a different path to use instead (would be required when cake.recipe support .NET Core only builds).

[gep13]

@AdmiringWorm this sounds like a good idea to me. Is this something that you wanted to take a stab at?

[AdmiringWorm]

Were you wanting to take a stab at this?

Yeah, most likely, either during the weekend or sometime next week.
I didn't want to assign myself until I start working on it, as it should be quite simple to implement, and I want to allow other people the opportunity to add the feature if they wish to help out.