How to modify service variable

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1

I’d like to insert an extra line in installPhase in the pkg derivation in this service:

https://github.com/NixOS/nixpkgs/blob/0ae51edb29dc6aa5f86c1e3686047032818af089/nixos/modules/services/web-apps/dokuwiki.nix#L36-L58

Is there a good way to do this?