porthos: users: add 'ambroisie' to 'media'

This only done when the groups exists.

machines/porthos/users.nix

@@ -1,7 +1,12 @@
 # User setup
-{ config, ... }:
+{ config, lib, ... }:
 let
   my = config.my;
+  groupIfExists = grp:
+    lib.lists.optional
+      (builtins.hasAttr grp config.users.groups)
+      grp;
+  groupsIfExist = grps: builtins.concatMap groupIfExists grps;
 in
 {
   users.users.blog = {
@@ -12,4 +17,8 @@ in
     home = "/var/www/";
     openssh.authorizedKeys.keys = [ my.secrets.drone.ssh.publicKey ];
   };
+
+  users.users.ambroisie.extraGroups = groupsIfExist [
+    "media"
+  ];
 }