VMR: move DevVersions jobs to a separate stage

[akoeplinger]

Don't run the validation jobs during the official build.

Fixes dotnet/source-build#4769

[ViktorHofer]

we only care about running them in sdk CI

We want to run that in the dotnet/dotnet CI (PRs and rolling builds) as well. Just not in official builds.

[akoeplinger]

ah right, do we have a good property for that? aren't all rolling builds in the VMR "official builds"? or are you talking about public vs internal pipelines?

[ViktorHofer]

Not sure if we have a property for it. Yes when I said rolling builds, I meant public ones. To be more precise I think these are the configurations when DevLegs should run:

• Public and internal PRs against dotnet/sdk or dotnet/dotnet
• Public rolling builds for dotnet/dotnet (which I think includes manually queued builds?) or IOW always in dotnet/dotnet except when performing official builds

[akoeplinger]

@ViktorHofer ok, fixed the conditions