So, the way allowUnfree specifically works is that it’s a property on the stdenv builder: nixpkgs/pkgs/stdenv/generic/check-meta.nix at 82deee3bfbf7dcb08ee8c3a1df2c1852101f49c9 · NixOS/nixpkgs · GitHub
As you can see, there’s a bit of a hack involved to read the environment variable. This of course is disabled in pure evaluation.
What you’re interested in is that config attrset which can be used alternatively, and since you’re using nix profile, specifically how to set it for an input flake.
The input flake for nix profile is the one that comes from your nix registry - this is importantly separate from the inputs of the flake you build your system with.
This is why setting allowUnfree on the config in configuration.nix doesn’t do anything here (that is also an anti-pattern by the way, you should be using the NixOS option) - even if you set your registry to use the same flake input, it would be evaluated from scratch, so the value of nixpkgs in your configuration.nix simply cannot affect whether unfree packages are allowed in nix profile.
The other way to change config is defined here: nixpkgs/pkgs/top-level/impure.nix at 82deee3bfbf7dcb08ee8c3a1df2c1852101f49c9 · NixOS/nixpkgs · GitHub
As importing from places outside of the flake is forbidden because that would be impure, this also cannot be used to change the settings in the context of nix profile. builtins.pathExists presumably evaluates to false if used on paths outside the flake, though this is a detail I’d also not realized before.
So there’s not really a good way to do this with flakes currently, this is one of their major limitations. There are proposals for adding flags to the input url and stuff, and suggestions to handle it some other way, but this hasn’t gone anywhere yet to my knowledge.
Until there’s a better solution, you can use GitHub - numtide/nixpkgs-unfree: nixpkgs with the unfree bits enabled. It simply takes its input nixpkgs (which you can either change by branch or through follows), modifies the config to allowUnfree, and re-exports everything. You’d need to set your entry for nixpkgs to it:
nix registry add nixpkgs github:numtide/nixpkgs-unfree/nixos-unstable
Or, alternatively, you can do so declaratively in your configuration.nix and set it to one of your flake inputs (in which case you would also need to add nixpkgs-unfree to your flake’s inputs, of course): dotfiles/nixos-config/default.nix at ef1a08cf30ab648f799c7369847fe935df7e7a93 · TLATER/dotfiles · GitHub
This additionally has the advantage that nix profile won’t randomly pull packages from a different version of nixpkgs than your system configuration does, because by default these are not synchronized - besides being frankly confusing, the default settings also add a delay to running nix profile every two hours.