os/exec: trivial allocation removal in LookPath

R=golang-dev, bsiegert, r
CC=golang-dev
https://golang.org/cl/5549043

src/pkg/os/exec/lp_unix.go

@@ -47,8 +47,9 @@ func LookPath(file string) (string, error) {
 			// Unix shell semantics: path element "" means "."
 			dir = "."
 		}
-		if err := findExecutable(dir + "/" + file); err == nil {
-			return dir + "/" + file, nil
+		path := dir + "/" + file
+		if err := findExecutable(path); err == nil {
+			return path, nil
 		}
 	}
 	return "", &Error{file, ErrNotFound}