cmd/go: fix build in airplane mode
cmd/go: fix build in airplane mode

LGTM=iant
R=golang-codereviews, adg, iant
CC=golang-codereviews
https://codereview.appspot.com/122190043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go