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>

misc/android/go_android_exec.go

@@ -158,6 +158,7 @@ func runMain() (int, error) {
 		`; export GOROOT="` + deviceGoroot + `"` +
 		`; export GOPATH="` + deviceGopath + `"` +
 		`; export CGO_ENABLED=0` +
+		`; export GOPROXY=` + os.Getenv("GOPROXY") +
 		`; export GOCACHE="` + deviceRoot + `/gocache"` +
 		`; export PATH=$PATH:"` + deviceGoroot + `/bin"` +
 		`; cd "` + deviceCwd + `"` +