misc/android: pass on GOPROXY to device environment

Android devices don't have git available, so many go tool commands
that fetch dependencies fail. Builders already have a GOPROXY
available, so pass that along to the device environment.

Updates #30885

Change-Id: Id0d2338932f0cd7de4d95d9e0ca9b79d29336ffe
Reviewed-on: https://go-review.googlesource.com/c/go/+/168118


TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>