Add command line option or environment variable to disable workload message

### 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