MacOS: add user to the trusted user list

If you have just installed Nix for the first time, you will most likely see the following error

✖ You're not a trusted user of the Nix store. You have the following options:

a) Add yourself to the trusted-users list in /etc/nix/nix.conf for devenv to manage caches for you.

trusted-users = root YOUR_USERNAME

Restart nix-daemon with:

  $ sudo launchctl kickstart -k system/org.nixos.nix-daemon

b) Add binary caches to /etc/nix/nix.conf yourself:

extra-substituters = https://devenv.cachix.org
extra-trusted-public-keys = devenv.cachix.org-1:w1cLUi8dv3hnoSPGAuibQv+f9TZLr6cv/Hm9XgU50cw=

And disable automatic cache configuration in `devenv.nix`:

{
    cachix
    .enable = false;
}

We recommend adding yourself as a trusted user in /etc/nix/nix.conf. Don't forget to restart the nix-daemon after doing this.