Nixpkgs cache

**Describe the bug** <!-- A clear and concise description of what the bug is. --> 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** <!-- A clear and concise description of what you expected to happen. --> 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** <!-- What version of the software. On which system. --> MacOS 15.4.1 and latest devshell. **Additional context** <!-- Add any other context about the problem here. -->