cmd/api: don't depend on os/user or USER to check api

The -nocgo builder failed because it has cgo disabled
as well as no USER environment variable:
http://build.golang.org/log/2250abb82f5022b72a12997b8ff89fcdeff094c9

# Checking API compatibility.
Error getting current user: user: Current not implemented on linux/amd64
exit status 1

Don't require the environment variable here.

LGTM=minux
R=dave, adg, minux
CC=golang-codereviews
https://codereview.appspot.com/140290043