Nixpkgs cache #348

Describe the bug

First off, I'm pretty new to Nix, so I might be getting this wrong.

When I run nix-shell -p elmPackages.elm-format, it instantly executed because it uses a cached artifact from cache.nixos.org.
When I put the same package in a devshell.toml however, it starts building everything from scratch including cabal, ghc, and whole host of other stuff.

To Reproduce

Steps to reproduce the behavior:

1. Add elmPackages.elm-format to your devshell.toml.
2. Watch it rebuild everything.

Expected behavior

I'd expect devshell to use the same cache. Maybe I'm holding it wrong, in which case I'd appreciate some help configuring it as such.

System information

MacOS 15.4.1 and latest devshell.

Additional context