flow through version props in a more idomatic way

[baronfel]

I was trying to make a prerelease package over in #894 and ran into a few stumbling blocks. This work unblocked me and made use of the project properties in a way that was easy to integrate with the FAKE script. I had to add the nowarn because the prerelease version property threw a nuget targets error:

    1>/Users/chethusk/.local/share/dotnetcore/sdk/2.1.504/NuGet.targets(114,5): error : '27.1.1-beta.001' is not a valid version string. [/Users/chethusk/oss/FSharp.Compiler.Service/fcs/FSharp.Compiler.Service.sln]
/Users/chethusk/.local/share/dotnetcore/sdk/2.1.504/NuGet.targets(114,5): error : Parameter name: value [/Users/chethusk/oss/FSharp.Compiler.Service/fcs/FSharp.Compiler.Service.sln]

As an aside, should changes like this go in this repository or in visualfsharp?

[Olivierdub38]

fcs/build.fsx
fcs/fcs.props