build: fix to use root modules when adding options

[RossComputerGuy]

Makes zon2nix work with current zig master.

[adamcstephens]

@figsoda any chance you can look at this? zon2nix is broken with zig 0.12 for building and parsing. I've verified @RossComputerGuy has all the fixes to add 0.12 support, and this is one of them. Thanks!