example usage: - nix-env -I nixpkgs=../somewhere/nixpkgs -f default.nix -iA emscriptenPackages.libxml2 - nix-env -I nixpkgs=../somewhere/nixpkgs -f default.nix -iA emscriptenPackages.json_c - nix-env -I nixpkgs=../somewhere/nixpkgs -f default.nix -iA emscriptenPackages.zlib - nix-env -I nixpkgs=../somewhere/nixpkgs -f default.nix -iA emscriptenPackages.xmlmirror