virtme-ng-init: add default secure_path to /etc/sudoers

When executing vng as a non-privileged user without /sbin in their
$PATH, attempting to run a command located in that directory with sudo
will not succeed.

Fix by defining a proper secure_path in /etc/sudoers.

The same change has been applied to virtme-init in the virtme-ng
repository, see commit:

 ac29432 ("virtme-init: Add default secure_path")

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

src/main.rs

@@ -307,10 +307,11 @@ fn generate_shadow() -> io::Result<()> {
 fn generate_sudoers() -> io::Result<()> {
     if let Ok(user) = env::var("virtme_user") {
         let fname = "/tmp/sudoers";
-        let content = format!(
-            "root ALL = (ALL) NOPASSWD: ALL\n{} ALL = (ALL) NOPASSWD: ALL\n",
-            user
-        );
+        let content = "Defaults secure_path=\"/usr/sbin:/usr/bin:/sbin:/bin\"\n".to_string()
+            + &format!(
+                "root ALL = (ALL) NOPASSWD: ALL\n{} ALL = (ALL) NOPASSWD: ALL\n",
+                user
+            );
         utils::create_file(fname, 0o0440, &content).ok();
         utils::do_mount(fname, "/etc/sudoers", "", libc::MS_BIND as usize, "");
     }