Only execute dotnet build-server shutdown at the end of official builds

[artidoro]

Currently we run dotnet build-server shutdown at the end of all Windows builds.

machinelearning/run.cmd

Lines 27 to 29 in dd1a922

	:: Terminate all dotnet build processes.
	call "%_dotnet%" "build-server" "shutdown"
	popd

As pointed out by @eerhardt in #1390 we should instead only run it at the end of official builds, by adding the command to the .yml file instead of run.cmd.