fix(metadata): detect login user via logname when running under sudo

On Darwin 26.2+, sudo no longer preserves SUDO_USER, LOGNAME, and USER
(CVE-2025-43416), so env-based detection fails. Use the logname(1)
command on Unix first, then fall back to environment variables and
user.Current() so the real login user is still reported correctly.

metadata_test.go

@@ -2,10 +2,14 @@ package ctrld
 
 import (
 	"context"
+	"os"
 	"testing"
 )
 
 func Test_metadata(t *testing.T) {
 	m := SystemMetadata(context.Background())
 	t.Logf("metadata: %v", m)
+	t.Logf("SUDO_USER: %s", os.Getenv("SUDO_USER"))
+	t.Logf("LOGNAME: %s", os.Getenv("LOGNAME"))
+	t.Logf("USER: %s", os.Getenv("USER"))
 }