flake.nix

{
  description = "Langusta - link aggregator in Elixir";

  inputs.nixpkgs.url = "flake:nixpkgs";
  inputs.flake-parts.url = "github:hercules-ci/flake-parts";

  inputs.devenv = {
    url = "github:cachix/devenv/python-rewrite";
    inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = {
    flake-parts,
    devenv,
    ...
  } @ inputs:
    flake-parts.lib.mkFlake {inherit inputs;} {
      systems = [
        "x86_64-linux"
      ];

      perSystem = {
        self',
        inputs',
        pkgs,
        ...
      }: {
        devShells = let
          mkShell = args: devenv.lib.mkShell ({inherit inputs pkgs;} // args);
        in {
          default = mkShell {
            modules = [
              {
                env.ABC = "123";
              }
            ];
          };
        };
      };
    };
}

shell output

❯ nix develop --impure
(devenv) 
[chrism@optinix:~/tmp/hauleth]$ env|grep ABC
ABC=123
(devenv) 
[chrism@optinix:~/tmp/hauleth]$