fix: widen make_rule cfg gate to include Linux

make_rule was gated to macOS-only but discover_linux() calls it for
search domain forwarding rules. CI failed on Linux with E0425.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/system_dns.rs

@@ -153,7 +153,7 @@ fn discover_macos() -> SystemDnsInfo {
     }
 }
 
-#[cfg(target_os = "macos")]
+#[cfg(any(target_os = "macos", target_os = "linux"))]
 fn make_rule(domain: &str, nameserver: &str) -> Option<ForwardingRule> {
     let addr: SocketAddr = format!("{}:53", nameserver).parse().ok()?;
     Some(ForwardingRule {