With my-geospatial-nix-flake you can run

nix profile install github:imincik/my-geospatial-nix-flake#qgis

to install QGIS with Flask (from nixpkgs) and Fiona (from geonix).

Or you can run it without installation

nix run github:imincik/my-geospatial-nix-flake#qgis