Based on this comment https://github.com/NixOS/nix/issues/7165#issuecomment-1309220847 and https://www.tweag.io/blog/2022-09-22-rust-nix/

Define a package all:

packages.all = pkgs.symlinkJoin {
  name = "all";
  paths = [ packages.a packages.b packages.c ];
};
default = packages.all;