update.nix was a huuuuge hack, abusing checksum collisions, etc., and
was extremely difficult to read and maintain, especially because
values from update.nix were also used in the derivations themselves!
I've replaced this with an implementation in Python, which I chose for
readability. Rather than generating Nix, I chose to
generate JSON, since Python can do that in the standard library and
Nix can read it.
I also set update.py as an updateScript, so Chromium can now
automatically be updated!
Fixes: https://github.com/NixOS/nixpkgs/issues/89635
de69b705d2