After some back and forth with JoyPixels they agreed to creating a
version of their font for macOS that does not use the exact same name as
the Apple Color Emoji default font.
This naming collision meant it was impossible to configure applications
to use the JoyPixels emoji font unless you disabled the Apple Color
Emoji font using Font Book. Which meant the JoyPixels font could either
replace the Apple Color Emoji font completely or only fill in the gaps
in that font (on my system "hot face" isn't in the system font) but not
be used entirely for specific apps or be used with the system font as a
back up.
By moving the assert concerning license acceptance into the src
attribute license acceptance can be expressed with an override,
`joypixels.override { acceptLicense = true; }`.
Specifying the system-specific variables for x86_64-darwin and
x86_64-linux is too restrictive, excluding for example i686-linux.
Since macOS seems to be the odd one out we can special-case only
x86_64-darwin.
The Arch Linux endpoint was not intended to be used as-is by other
distros. I have asked for and received a proper license with a dedicated
endpoint for NixOS.
