Ability to suppress welcome message

The "Welcome to .NET Core 3.0!.. " spiel adds considerable noise to CI logs.

While we can work around this by creating a file named "%HOME%/.dotnet/$(dotnet --version).dotnetFirstUseSentinel", this is hacky and adds noise to bulid configs.

It would be better to have a CLI option to suppress the message, e.g.

dotnet build --no-welcome

or

dotnet build --skip-first-time-experience

etc.

[am11]

before dotnet-cli 3.0:

export DOTNET_SKIP_FIRST_TIME_EXPERIENCE=1
dotnet new console

(aligns with other knobs DOTNET_CLI_TELEMETRY_OPTOUT etc.; many users including AppVeyor CI service were relying on it to keep the build job outputs clean)

in 3.0 (since 3.0 RC2: https://github.com/dotnet/cli/commit/7efbf4770630fd370779d222ce9aba35ffe56100):

touch "$HOME"/.dotnet/"$(dotnet --version)".dotnetFirstUseSentinel
dotnet new console

i think it would be just fine if DOTNET_SKIP_FIRST_TIME_EXPERIENCE is back in 3.1.

[sfoslund]

Solved by dotnet/cli#13275