The ntfy package expects to have pyobjc available when running under Darwin,
which is currently broken in nixpkgs. There's a fairly involved ongoing effort
to package it again, but in the mean time we just patch out the dep. I'm using
the pushover backend anyway.
To avoid having to rebuild it rather than just fetch from the NixOS cache, I
only override it when running on Darwin.
I'm assuming (probably wrongly) that anything using my graphical-client preset
is _also_ using NetworkManager, which is probably true for real client machines
but may not be true on terminal services machines which also end up with this
preset.
Whatever, I'll work it out later.
To allow using my home-manager config on Darwin (and other non-NixOS
machines), I introduce the concept of home-manager-ext, which gives
me a much easier hook to import my config elsewhere.
