nixos/dhcpcd: fix updating resolv.conf when using systemd-resolved

Fix the regression between NixOS 24.05 and 24.11 where using dhcpcd
(e.g. networking.useDHCP) and systemd-resolved
(services.resolved.enable) result in no "search" entry getting added to
/etc/resolv.conf, and dhcpcd logging the following error:

  $ systemctl status dhcpcd
  [...] dhcpcd[2896]: Failed to set DNS configuration: Interactive authentication required.

Fix it by adding a polkit rule that gives the required permissions to
the 'dhcpcd' user to manipulate resolved. The rule was made by using
polkit logging and allowing each action.id until the above error went
away, and /etc/resolv.conf got the correct search entry.

nixos/modules/services/networking/dhcpcd.nix

@@ -303,6 +303,18 @@ in
         /run/current-system/systemd/bin/systemctl reload dhcpcd.service
       '';
 
+    security.polkit.extraConfig = lib.mkIf config.services.resolved.enable ''
+      polkit.addRule(function(action, subject) {
+          if (action.id == 'org.freedesktop.resolve1.revert' ||
+              action.id == 'org.freedesktop.resolve1.set-dns-servers' ||
+              action.id == 'org.freedesktop.resolve1.set-domains') {
+              if (subject.user == '${config.systemd.services.dhcpcd.serviceConfig.User}') {
+                  return polkit.Result.YES;
+              }
+          }
+      });
+    '';
+
   };
 
 }