This fixes the Nix *package* build (read: not devShell, which is not
touched) so that it builds, and also conforms to what is generally seen
for a Zig project in nixpkgs.
The highlights:
* We use a Zig 0.12 derivation that I constructed from the Zig 0.11
derivation, in addition to LLVM 17 updates found in
NixOS/nixpkgs#258614. This specifically includes the patches that
address ziglang/zig#15898, and also allows us to take advantage of the
build hooks included in the Zig toolchain there.
* We pre-download the cache using "zig build --fetch" and a fixed-output
derivation. This is similar to how the Go builders work in nixpkgs and
I could see Zig ultimately going in a similar path, given that the
fetcher part of the build system seems to be shaping up to having a Go
module system-style DX (mind you, this is a naive opinion right now).
* Finally, cleaned up the derivation so that there's no special fixups
happening outside of what is defined in the basic nixpkgs workflow.
This is similar to other Zig projects I looked at (notably River) that
seem to just include their dependencies in buildInputs and call it a
day.
One specific change that is worth noting is that this changes the build
mode from ReleaseFast to ReleaseSafe - this is the current default
within the Zig build hook in nixpkgs. If we need ReleaseFast, we'll have
to override this.
