Add command line option or environment variable to disable workload message

[duksway]

The Challenge

When running dotnet build, I commonly see this message:
Workload updates are available. Run dotnet workload list for more information.
This message is helpful, but it would also be useful if it could be hidden to simplify build output

Proposed solution

This could be hidden by either:

1. A command line option for dotnet.exe
2. An environment variable