attic-server: fix faulty meta attrs override for mainProgram

pkgs/by-name/at/attic-server/package.nix

···
-{ attic-client
-}:
-(attic-client.override {
-  crates = [ "attic-server" ];
-}).overrideAttrs {
-  meta.mainProgram = "atticd";
-}
+{ lib, attic-client }:
+lib.addMetaAttrs { mainProgram = "atticd"; } (
+  attic-client.override { crates = [ "attic-server" ]; }
+)