nixos/mastodon: set environment for mastodon-init-dirs (#112917)

mastodon-init-dirs should have the same environment as the other
services, as it also calls rake. Especially RAILS_ENV was missing.

nixos/modules/services/web-apps/mastodon.nix

@@ -367,6 +367,7 @@ in {
       '' else "") + ''
         EOF
       '';
+      environment = env;
       serviceConfig = {
         Type = "oneshot";
         User = cfg.user;