Pre-initialize .NET Core

[derwasp]

So the build slaves will not initialize it each time.

Same as here:
derwasp/jenkins-jnlp-dind#2

[derwasp]

https://github.com/dotnet/cli/issues/4919

[derwasp]

added dotnet new and moved latest tags with
git tag -l --contains HEAD~1 | %{ 'git tag --force ' + $_ + ' ' + (git rev-parse HEAD) + ' && git push --force --tags'}