In the outputs section of your flake.nix. write something like fhs_env = pkgs.buildFHSUserEnv {...};. Then you can run it with nix run .#fhs_env. If your runScript is a shell, you’ll enter the shell where you can explore the environment to determine what to adjust. See the wiki page on Flakes for details on the structure of flake.nix. Remember that flake.nix and flake.lock must be part of a git repo to use the flake.
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