Better error message when `nix` is not installed

[loreto]

Nix is a pre-requisite to using devbox. When missing or not installed properly, devbox add fails with:

Error: Package fddfdf not found.

Which makes it sound like the package does not exist. Improve error message to explain that nix should be installed.

[loreto]

We just released a new version of devbox that should address this issue. If you installed via our install script, you should get the new version automatically next time you run the binary.