phone: Add mullvad support by disabling checkReversePath

`networking.firewall.checkReversePath` was being set to "loose" from
Mullvad VPN, which was causing an issue with the kernel used by the
PinePhone with Mobile NixOS.

By changing this option to `false`, we get rid of the "This kernel does
not support rpfilter" error, which seems to be inaccurate due to the
result of `sysctl -a | grep \\.rp_filter` on the phone being consistent
with the result on the laptop.

hosts/phone/configuration.nix

@@ -27,6 +27,10 @@ in
       phone = true;
       sensor = true;
     };
+
+    system = {
+      mullvad = true;
+    };
   };
 
   environment = {

modules/system.nix

@@ -227,6 +227,8 @@ in
         allowedTCPPorts = mkIf allowDevPort [
           3000
         ];
+
+        checkReversePath = mkIf phone (lib.mkForce false);
       };
     };
 
@@ -236,6 +238,7 @@ in
       mullvad-vpn = mkIf mullvad {
         enable = true;
         enableExcludeWrapper = false;
+        package = pkgs.mullvad-vpn;
       };
 
       postgresql = mkIf postgres {