Updates build.zig with to use new APIs

[jonathannen]

Updates the build.zig file to use some new APIs. Most of the changes were a reference (which was never used anyway) and changing how the path fields work.

I've tested this end-to-end on a MacOS build using zig 0.13.0 with the shell.zig working at the end.

See: https://github.com/fubark/zig-v8/issues/16