Refactor vmr-build job

[NikolaMilosavljevic]

There are multiple direct invocations of build.sh or build.cmd, in unified-build templates.

Build and test steps should be refactored out of job/vmr.build.yml into a separate template, that can be consumed from those other invocation places, i.e. https://github.com/dotnet/sdk/blob/0ec093d6b696ec894beec76c415dbfe9b1c8df60/eng/pipelines/templates/steps/vmr-validate-installers.yml#L34