fix: fix "Nix" step to use nix upgrade-nix in more situations (#550)

`nix upgrade-nix` can be used on any platform except NixOS where `nix`
is available.

Also use `nix profile upgrade --verbose` because the non-verbose mode
doesn't print anything on stdout.

src/steps/git.rs

@@ -170,7 +170,7 @@ impl Git {
                 }
             }
             Err(e) => match e.kind() {
-                io::ErrorKind::NotFound => debug!("{} does not exists", path.as_ref().display()),
+                io::ErrorKind::NotFound => debug!("{} does not exist", path.as_ref().display()),
                 _ => error!("Error looking for {}: {}", path.as_ref().display(), e),
             },
         }