Update installation documentation for Mac OS

[mackensen]

Describe the bug
The experimental Nix installer changed its packaging approach in NixOS/experimental-nix-installer@d028660 so downloading and invoking nix-installer.sh now causes this error:

sh: line 28: assemble_installer_templated_version: unbound variable

Using the default from that project produces a working environment:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix | \
  sh -s -- install