core: custom entrypoint build API design broken after Zig 2024.1.0-mach update

[slimsag]

After the recent Zig update #1135 it is no longer possible to specify .main_mod_path. We relied on this when letting users specify a custom entrypoint.

There is probably a workable solution here by revisiting the entire design of mach-core and mach engine's build API design, but it will take some time to sort out. Until then custom entrypoints may not work.