nixos-rebuild: restore local sudo functionality

This functionality used to exist and was then removed in #331741. The
issue was that $SUDO_USER is already used by `sudo` as an environment
variable.

This change makes it possible to checkout a flake repository, as a user,
enter the devshell, and with the additions below and apply changes with
`nixos-rebuild switch`.

```nix
{ pkgs }:
let
  nixos-rebuild = pkgs.writeShellApplication {
    name = "nixos-rebuild";
    runtimeInputs = [ pkgs.nixos-rebuild ];
    text = ''
      set -euo pipefail
      exec nixos-rebuild --flake "$PRJ_ROOT" --use-local-sudo "$@"
    '';
  };
in
pkgs.mkShellNoCC {
  packages = [
    nixos-rebuild
  ];

  shellHook = ''
    export PRJ_ROOT=$PWD
  '';
}
```

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

@@ -33,6 +33,7 @@ imageVariant=
 buildHost=
 targetHost=
 remoteSudo=
+localSudo=
 noSSHTTY=
 verboseScript=
 noFlake=
@@ -173,6 +174,9 @@ while [ "$#" -gt 0 ]; do
       --use-remote-sudo)
         remoteSudo=1
         ;;
+      --use-local-sudo)
+        localSudo=1
+        ;;
       --no-ssh-tty)
         noSSHTTY=1
         ;;
@@ -256,7 +260,7 @@ targetHostSudoCmd() {
         t="-t"
     fi
 
-    if [ -n "$remoteSudo" ]; then
+    if [[ -n "$remoteSudo" || -n "$localSudo" ]]; then
         useSudo=1 SSHOPTS="$SSHOPTS $t" targetHostCmd "$@"
     else
         # While a tty might not be necessary, we apply it to be consistent with