- update nixpkgs now that Zig 0.15.2 is available in nixpkgs
- drop hack that worked around compile failures on systems with more
than 32 cores
- enforce patch version of Zig
The SDK published on Flathub updated to Zig 0.15.1 which broke the
Flathub build in CI. So let's install it ourselves so that we can
control the version.