nixos/network-interfaces: assertion for DHCP on bridges

Assert that the user doesn't have a bridge configured while
networking.useDHCP is true. Due to new behaviour of dhcpcd [0], this
would result in the bridge not getting an address via DHCP, regardless
of wether it has networking.interfaces.<name?>.useDHCP set or not.

[0] https://roy.marples.name/archives/dhcpcd-discuss/0002621.html

nixos/modules/tasks/network-interfaces.nix

@@ -976,6 +976,17 @@ in
           Temporary addresses are only needed when IPv6 is enabled.
         '';
       })) ++ [
+        {
+          assertion = cfg.useDHCP -> cfg.bridges == {};
+          message = ''
+            There are the bridges [ ${builtins.toString (builtins.attrNames cfg.bridges)} ] configured while `networking.useDHCP` is enabled.
+            dhcpcd doesn't give IPv4 addresses to bridges by default anymore,
+            so you have to set `networking.useDHCP = false` and then whitelist
+            every interface you need DHCP on with
+            `networking.interfaces.<name?>.useDHCP = true`.
+          '';
+        }
+      ] ++ [
         {
           assertion = cfg.hostId == null || (stringLength cfg.hostId == 8 && isHexString cfg.hostId);
           message = "Invalid value given to the networking.hostId option.";