misc/bash/go: Add a completion rule for "go env".

"env" is a valid go command. This patch is to make bash to autocomplete it.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/74660045

misc/bash/go

@@ -20,7 +20,7 @@ _go()
 
   local cmd="${COMP_WORDS[1]}"
 
-  local cmds="build clean doc fix fmt get
+  local cmds="build clean env doc fix fmt get
     install list run test tool version vet"
   local addhelp="gopath importpath remote
     testflag testfunc"