nixos/systemd: fix run0 failing to run commands

Fixes #361592.

I was able to test this change by doing the following:

1. Create a file named “test-systemd-run0.nix” that contains this Nix
expression:

    let
      nixpkgs = /path/to/nixpkgs;
      pkgs = import nixpkgs { };
    in
    pkgs.testers.runNixOSTest {
      name = "test-systemd-run0";
      nodes.machine = {
        security.polkit.enable = true;
      };
      testScript = ''
        start_all()
        machine.succeed("run0 env")
      '';
    }

2. Replace “/path/to/nixpkgs” with the actual path to an actual copy of
Nixpkgs.

3. Run the integration test by running this command:

    nix-build <path to test-systemd-run0.nix>

nixos/modules/system/boot/systemd.nix

@@ -832,6 +832,15 @@ in
         minsize = "1M";
       };
     };
+
+    # run0 is supposed to authenticate the user via polkit and then run a command. Without this next
+    # part, run0 would fail to run the command even if authentication is successful and the user has
+    # permission to run the command. This next part is only enabled if polkit is enabled because the
+    # error that we’re trying to avoid can’t possibly happen if polkit isn’t enabled. When polkit isn’t
+    # enabled, run0 will fail before it even tries to run the command.
+    security.pam.services = mkIf config.security.polkit.enable {
+      systemd-run0 = { };
+    };
   };
 
   # FIXME: Remove these eventually.