src/arcade/global.json must always be updated as part of a re-bootstrap

[ViktorHofer]

More details in dotnet/runtime#115149 but the gist is that tools produced in VMR repos encode the SDK version in their runtimeconfig.json (as they target that version) and when the VMR re-bootstraps and only updates the VMR's bootstrap SDK version (under /global.json) but not the "outgoing/flowing" (under src/arcade/global.json) version, repositories who invoke tools produced in the VMR will get "You must install or update .NET to run this application." errors.

IOW the SDK version used to build i.e. arcade in the VMR must be the one that flows back into the repos.

[ViktorHofer]

So, given that the entire VMR builds with the bootstrap SDK (regardless of what's defined in src/arcade/global.json), I think it would make sense to use global.json from the VMR root as the input to backflow into the repos.

@mmitche opinions? cc @premun

[ViktorHofer]

If we do this, we should probably update https://github.com/dotnet/arcade/blob/d81c32622b8be2deeb76986a7cca09d84a5418ba/src/Microsoft.DotNet.Helix/Sdk/tools/dotnet-cli/DotNetCli.props#L6 to use the bundled runtime version as aspnetcore and runtime produced from the VMR will have the same version. Not great from a contract perspective but avoids the hardcoded value.