docs: Add instructions to use cachix on nixos

[k3yss]

While compiling devenv on NixOS, I noticed that the instructions for using cachix were only provided for non-NixOS distributions. This PR addresses this issue and adds documentation for NixOS.

[domenkozar]

If it's NixOS we can check if /etc/NIXOS exists and then offer append the extra instructions.

[k3yss]

Or we can simply not display the first instructions and only display OS specific instructions:

A simple bash script to demonstrate what I mean

if [ -f /etc/NIXOS ]; then
    echo "Running on NixOS"
    # Perform NixOS-specific tasks here
else
    echo "Not running on NixOS"
    # Perform tasks for other systems
fi