feat: add nix demo

__linux/nix-demo/src/main.rs

@@ -0,0 +1,14 @@
+use nix::unistd::{getgid, gethostname, getpid, getuid};
+
+fn main() {
+    let hostname = gethostname().unwrap();
+    println!("Hostname: {}", hostname.to_string_lossy());
+
+    let pid = getpid();
+    println!("Pid: {}", pid);
+
+    let uid = getuid();
+    println!("Uid: {}", uid);
+    let gid = getgid();
+    println!("Gid: {}", gid);
+}