Add flag to set the .NET version

[dansiegel]

In CI when using a cloud hosted agent, the agent may be updated to a newer version of .NET than what Uno.Check is installing the workloads to. This results in builds that may fail without warning even though uno-check completes successfully.

It would be useful to have a flag that could be set so that in CI uno-check can search recursively for any global.json files and update them so that the version of the .NET SDK is set forcing the build to use the version of .NET that uno-check is configuring the workloads for. This also will ensure reproducible CI builds without requiring that a specific version be set in the repository making it easier for people to clone repos locally.