cmd/go: write the WORK=/tmp/... line to stderr

Unlike the other output from the -x flag, it was going to stdout.
Fixes #6362.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13746044

src/cmd/go/build.go

@@ -435,7 +435,7 @@ func (b *builder) init() {
 			fatalf("%s", err)
 		}
 		if buildX || buildWork {
-			fmt.Printf("WORK=%s\n", b.work)
+			fmt.Fprintf(os.Stderr, "WORK=%s\n", b.work)
 		}
 		if !buildWork {
 			atexit(func() { os.RemoveAll(b.work) })