build: only print "You need to add foo to PATH" when needed

Fixes #1223.

R=bradfitz
CC=golang-dev
https://golang.org/cl/2701041

src/make.bash

@@ -96,7 +96,7 @@ installed() {
 	echo Installed Go for $GOOS/$GOARCH in "$GOROOT".
 	echo Installed commands in "$GOBIN".
 	case "$OLDPATH" in
-	*":$GOBIN" | *":$GOBIN:"*)
+	"$GOBIN:"* | *":$GOBIN" | *":$GOBIN:"*)
 		;;
 	*)
 		echo '***' "You need to add $GOBIN to your "'$PATH.' '***'