fabs no longer a zig builtin

[jeremy-coleman]

zig changed fabs to abs, which has broken dear pixi. I have fixed it and submitted a PR #38

hope its helpful, thanks dood

[foxnne]

Thank you so much for the PR, I've merged it and updated the readme to include a link to mach's mirror of the supported Zig version, which is the version Pixi will track.