cmd/go: 'go env' print GO15VENDOREXPERIMENT in Go1.5

Fixes #11410

Change-Id: I9d01be3e01f74f4b12a516aba8a5d20d9b277ec6
Reviewed-on: https://go-review.googlesource.com/11539Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/env.go

@@ -44,6 +44,7 @@ func mkEnv() []envVar {
 		{"GORACE", os.Getenv("GORACE")},
 		{"GOROOT", goroot},
 		{"GOTOOLDIR", toolDir},
+		{"GO15VENDOREXPERIMENT", os.Getenv("GO15VENDOREXPERIMENT")},
 
 		// disable escape codes in clang errors
 		{"TERM", "dumb"},