virtme-ng-init: fix build error with older versions of rustc

In older versions of rustc (such as 1.66.1) we can trigger the following
build error:

error[E0308]: mismatched types
   --> src/main.rs:632:20
    |
632 |             ("su", vec![&user, "-c", USER_SCRIPT])
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                    |
    |                    expected slice, found array `[&str; 3]`
    |                    arguments to this function are incorrect
    |
    = note: expected struct `Box<[&String], _>`
               found struct `Box<[&str; 3], std::alloc::Global>`

This happens becausee we are using a mixture of &String and &str types
with the vec! macro.

To prevent this make sure that the types match, by explicitly converting
user to &str when creating the vector.

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

Author: Andrea Righi

src/main.rs

@@ -629,7 +629,7 @@ fn run_user_script(uid: u32) {
         // Determine if we need to switch to a different user, or if we can run the script as root.
         let user = env::var("virtme_user").unwrap_or_else(|_| String::new());
         let (cmd, args) = if !user.is_empty() {
-            ("su", vec![&user, "-c", USER_SCRIPT])
+            ("su", vec![user.as_str(), "-c", USER_SCRIPT])
         } else {
             ("/bin/sh", vec![USER_SCRIPT])
         };