Use id command for user lookup on MacOS (#384)

When building client without CGO, user.Lookup
attempts to get user from /etc/passwd
Which doesn't have the user as MacOS uses
opendirectoryd as user directory

client/ssh/lookup.go

@@ -0,0 +1,10 @@
+//go:build !darwin
+// +build !darwin
+
+package ssh
+
+import "os/user"
+
+func userNameLookup(username string) (*user.User, error) {
+	return user.Lookup(username)
+}