Cleaning the nix store

Sometimes your store fills up the disk, but a simple `nix-store --gc` does not seem to clean all that much.

This usually means that you have some garbage collection roots that keep old versions of nixpkgs around.

There is multiple tools that can help with discovering these gc roots:

• https://github.com/symphorien/nix-du
• https://github.com/utdemir/nix-tree
• https://github.com/cdepillabout/nix-query-tree-viewer
• https://github.com/craigmbooth/nix-visualize