cmd/go: add go telemetry command and GOTELEMETRY, GOTELEMETRYDIR
Add the go telemetry command to support setting and viewing the
telemetry mode. Also add the non-settable GOTELEMETRY and GOTELEMETRYDIR
variables to go env, which contain the mode and telemetry dir.
For #67111
Change-Id: Id7e89cefe30acfe3d865fa467315fe7cda975de9
Reviewed-on: https://go-review.googlesource.com/c/go/+/584535 Reviewed-by: Robert Findley <rfindley@google.com> Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>