nixos-container: Use machinectl shell (#18825)

Using nsenter+su was suboptimal since the environment they provided was
not quite the same as the real container environment. See [1] for more
background.

Note that we directly invoke /bin/sh for `nixos-container root-login`
since setting the user's shell is quite complex. See [2] for a related
systemd issue.

[1]: https://github.com/systemd/systemd/issues/825#issuecomment-127917622
[2]: https://github.com/systemd/systemd/issues/1395

pkgs/tools/virtualization/nixos-container/default.nix

@@ -6,8 +6,6 @@ substituteAll {
     isExecutable = true;
     src = ./nixos-container.pl;
     perl = "${perl}/bin/perl -I${perlPackages.FileSlurp}/lib/perl5/site_perl";
-    su = "${shadow.su}/bin/su";
-    inherit utillinux;
 
     postInstall = ''
       t=$out/etc/bash_completion.d