An attempt at making opt-out telemetry easier to decline

I made this in response to the current Go telemetry issue, where the Go team wishes to add telemetry to the toolchain. After seeing this over and over and over again, I would like to propose a standard ENV that indicates that a user does not want to be tracked. Of course making this opt-in would be preferable for obvious reasons, but unfortunately that's just not the world we live in and this way at least would make opting out once and for all easier.