fix mono build/install

[dsyme]

@brettfo Looks like a typo. (I don't know why our CI build for Mono on Linux wasn't picking this up)

[dsyme]

See fsharp/fsharp#851 for the integration to fsharp/fsharp that found this

[dsyme]

@KevinRansom @brettfo This also reverts the change to use FSharpCoreFrozenPortablePackageVersion in the tests\projects\... and some others. This variable is not defined in these project files. The point of these projects is to make sure we continue to build old project files which don't use any of our variable/settings etc. (though in this case they do refer to a nuget package we have)

-      <HintPath>..\..\..\packages\Microsoft.Portable.FSharp.Core.$(FSharpCoreFrozenPortablePackageVersion)\lib\profiles\portable-net45+sl5+netcore45\FSharp.Core.dll</HintPath>
+      <HintPath>..\..\..\packages\Microsoft.Portable.FSharp.Core.10.1.0\lib\profiles\portable-net45+sl5+netcore45\FSharp.Core.dll</HintPath>

I could well imagine that change was the cause of many test failures in FCS and VisualFSharp unit tests....