Fix nix on multi-user setup (fix #646) (#650)

src/steps/os/unix.rs

@@ -175,7 +175,10 @@ pub fn run_nix(ctx: &ExecutionContext) -> Result<()> {
 
     if multi_user {
         if let Some(sudo) = ctx.sudo() {
-            run_type.execute(&sudo).arg("nix").arg("upgrade-nix").check_run()?;
+            run_type
+                .execute(&sudo)
+                .args(&["-i", "nix", "upgrade-nix"])
+                .check_run()?;
         } else {
             print_warning("Need sudo to upgrade Nix");
         }