How to create a editable config file inside the nvim directory

I have a file ~/.config/nvim/coderunner.vim which I want to be able to edit to change what command is ran for different languages. I want the file to be created and updated by home manager, but between these edits, I would like to be able to edit it manually (without the changes being reflected in my nix repo).

If home manager was to just create a file that was copied from inside my nix-repo instead of creating a symlink. How can I manage this? (if it was to just not create a symlink, but a copy, I would be happy, or if there is another and better way of achieving the same)

As far as I know, home-manager has no simple way to accomplish this, and my advice would be to seriously reconsider whether it has to be done this way. Can you find a way to separate the changeable part from the part you want home-manager to manage?

If you really must do this, I think the best you can do is an activation snippet.

I have made this quite bad plugin for neovim, which just has hard coded in what command is ran for the different languages, but sometimes, if I want to run a different command, I need to edit that file. I could of course just improve the plugin, but I kind of would like to be able to just keep my current setup