Inject `MSBUILDTERMINALLOGGER=off` into chat terminal sessions

[aelij]

Is your feature request related to a problem? Please describe.

When Copilot invokes dotnet build it needlessly gets a steam of unnecessary text due to .NET's terminal logger.

I wasn't sure if I should open this here or in the VS Code repo but as it is .NET-specific, opted for here.

Describe the solution you would like

Inject the above environment variable to Copilot terminal sessions to disable the terminal logger.

Applicable Scenarios

Copilot invoking .NET commands.