Setup nightly build

[MarcoRossignoli]

This PR setup nightly build, what we have:

• added Directory.Build.props with Nerdbank.GitVersioning ref for git versioning and setup of PackageOutputPath
• cleaned up coverlet.collector.csproj to use custom nuspec so we can also cleanup build.proj
• added simple ps script for upload packages eng/nightly.ps1 I know that we could use AZP to do all works(build artifacts and release)...but for now I prefer use what we have on master...and in future step by step we'll refactor build with @AArnott advices, pinned sdk for generic build and test with multi SDK locally testable.
  Script accept 2 params -apiKey xxx and -source https://... I tested all workflow on https://nugetservercoverlet.azurewebsites.net/ and it works well.
  @tonerdo you must setup variable with myget credentials like
  
  Here you can see tests log https://marcorossignoli.visualstudio.com/githubazp/_build/results?buildId=765
  YAML ignore errors, because if we don't have new commit on master donet nuget push fails for "package already present".
• pinned sdk on global.json fist step of @AArnott advice, this update does not conflicts with Make build more coherent with same "dotnet sdk" on all CI machine. #396 because there we use DOTNET_MULTILEVEL_LOOKUP=0
  BTW after this merge we'll have to override build.proj with new one, but I look forward to fix that

@toni after merge you need to create a new "Scheduled" pipeline(eng/azure-pipelines-nightly.yml) by hand like

Scheduled trigger is not yet supported with YAML https://docs.microsoft.com/en-us/azure/devops/pipelines/build/triggers?view=azure-devops&tabs=yaml#scheduled-triggers

To local test

dotnet msbuild build.proj // like prev
dotnet msbuild build.proj /p:Configuration=Release // like prev
dotnet msbuild build.proj /p:Configuration=Release /p:PublicRelease=false // generate gitversioning nuget package

Format will be coverlet.collector.1.0.0-g2478511111.nupkg where 2478511111 is the first n part of last commit. Using scripts https://github.com/AArnott/Nerdbank.GitVersioning/tree/master/src/Nerdbank.GitVersioning.Tasks/tools we can translate version to commit.

cc: @vagisha-nidhi

PS. A mega-huge thank's to @AArnott and @onovotny for the tool and help with setup, very appreciated!

[MarcoRossignoli]

@tonerdo after merge of this and new documentation I'll add "Consume nightly build" doc.

[tonerdo]

@MarcoRossignoli just tried to schedule a build. There's no way to specify eng/azure-pipelines-nightly.yml as the config to use

[MarcoRossignoli]

/azp run