CL 31330 added more envvars to "go env".
This CL brings them to "go bug" as well.

Change-Id: Iae122072c8178007eda8b765aaa3f38c3c6e39a0
Reviewed-on: https://go-review.googlesource.com/32011
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>