internal/testenv: update Builder docs with LUCI builder names

internal/testenv: update Builder docs with LUCI builder names

Also include some advice to avoid using this when possible. It makes
tests more prone to breaking under infrastructure changes.

Change-Id: Ifb2848742347eb0c937547dd888ab2cad3343f2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/703115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>