That’s not how nixpkgs config with flakes works, I happened to have written a deep-dive on it here: Permanently enabling unfree packages for nix profile (system config uses flake) - #2 by TLATER
Good spot however that aerweave is not built with an overlay, despite being added through an overlay, so we need to modify the configuration of the nixpkgs it is built with.
The easiest way would be to override the pkgs aerweave is built from, I suppose, but I’ve not tested this - unsure it will make stdenv.mkDerivation come from our pkgs:
environment.systemPackages = let
aerweave = pkgs.aerweave.override { inherit pkgs; }
in [
aerweave
];
Ultimately this is an upstream bug, though. Their project has not updated flake.lock in two years either, I’d ping them an issue.
I’d recommend you stick to adding the configuration via the NixOS module, by the way, and remove the nixpkgs.pkgs setting as well as the stuff you added in flake.nix.