Updated flake with separate stage0 and stage1

Stream: lean4

[Willem vanhulle]

Hey, are there any Nix users hacking on Lean4? If you want, you can use my updated flake.nix file. It separates stage0 as a Nix build target and uses the Nixpkgs provided mimalloc. This might make it faster to iterate on changes to the Lean code in the Lean4 Git repo while you are using Nix.