IIRC flake evaluation caching doesn’t work for nested package sets (as nixpkgs is) and thus is not disabled for anything that is in legacyPackages (normal packages has to be a flat attribute set).
On a more general note, I don’t think that plan for opam will be very fruitful – installing has a very different sense on NixOS to other linux distributions. Specifically depext for libraries won’t work, as NixOS doesn’t have something akin to /usr/lib where linkers will look for libraries, so not all depext will work as expected – or rather only depext that just want to make an executable available may work. (I seem to remember nixos depext where added as a way for opam2nix to track some needed metadata in the opam repository.)
Overall the use of nix-env has some pitfalls and there is a risk of messing up an users environment when invoking it in this way. If I were to add a Nix integration to opam, I’d rather have it set up a switch specific nix-shell environment and execute build commands within that environment (which of course would be a very different approach to other depext integration, but I believe necessarily so).