nixos/matrix-appservice-irc: fix chown of registration.yml in pre-script

Before the startup, the matrix-appservice-irc service sets up the
registration file such that it can be used by matrix-synapse. Part of
that setup requires us to change the group of said file so that the home
server can read it. Consequently, we need CAP_CHOWN and require that the
@chown system calls are allowed.

While we supposedly set up both of these, the setup of system calls is
broken as we have both an allow and a deny list of syscalls. But while
the allow list contains "@chown", the deny list contains "@privileged"
which contains "@chown" itself. So ultimately, we end up denying
"@chown".

Fix this issue by specifying "@chown" after the deny list.

nixos/modules/services/matrix/appservice-irc.nix

@@ -214,8 +214,9 @@ in {
         RestrictRealtime = true;
         PrivateMounts = true;
         SystemCallFilter = [
-          "@system-service @pkey @chown"
+          "@system-service @pkey"
           "~@privileged @resources"
+          "@chown"
         ];
         SystemCallArchitectures = "native";
         # AF_UNIX is required to connect to a postgres socket.