Run dotnet pack in CI too

[Smaug123]

ref: #663

Seems reasonable to fail CI if we can't create the NuGet package locally.

This has some interaction I don't understand with AppVeyor, which has uploaded artefacts… somewhere. I hope I haven't somehow blatted prod, but this way the nupkgs are available at https://ci.appveyor.com/project/kurtschelfthout/fscheck/builds/49266158/artifacts (for example).

Collecting artifacts...
Found artifact 'bin\FsCheck.3.0.0-rc2.nupkg' matching 'bin\*.nupkg' path
Found artifact 'bin\FsCheck.NUnit.3.0.0-rc2.nupkg' matching 'bin\*.nupkg' path
Found artifact 'bin\FsCheck.Xunit.3.0.0-rc2.nupkg' matching 'bin\*.nupkg' path
Uploading artifacts...
[1/3] bin\FsCheck.3.0.0-rc2.nupkg (379,992 bytes)...100%
[2/3] bin\FsCheck.NUnit.3.0.0-rc2.nupkg (15,610 bytes)...100%
[3/3] bin\FsCheck.Xunit.3.0.0-rc2.nupkg (27,565 bytes)...100%

[kurtschelfthout]

his has some interaction I don't understand with AppVeyor, which has uploaded artefacts…

Yep, that's this: https://github.com/fscheck/FsCheck/blob/master/appveyor.yml#L13

Doesn't go to nuget, so that's fine.