NUR upstream requires that everything builds on Nixpkgs unstable, so I
guess that's what we're using. Also enable NixOS unstable to see if we
can get that for free as well.
Ones of the changes from 0.5.0 is depending on the system fontconfig,
and one of the ligature changes forces the use of the system freetype.
One of the Rust libraries depends on the C++ stdlib.
At some point the build process stopped bundling the OpenGL libraries,
so add those back in too.
The old method was having the alacritty build copy-pasted into the repo
and making minor changes. This worked but isn't really feasible as it
doesn't incoroprate upstream (build) changes without noticing.
This commit finally works out how to override buildRustPackage to set
the Cargo dependency.
