Compiling from source: reproducible builds using nix

[arunoruto]

Hi! I just recently gave taichi a go and was really amazed how good it worked! So thanks for you hard work 💪🏻

Since I am a Nix user, or more specifically NixOS, I wanted to package taichi and make it available on nixpkgs.
I had two options:

1. Fetch the appropriate wheel from PyPI and patch it for nix, or
2. Build from source right away!

My first attempt was to package it from pypi, but taichi diverts a bit from the usual naming conveniention and I had to specify the wheel URLs and hashes manually, which is a bit cumbersome. Also, AMDGPU support is only available if compiled with LLVM from ROCm.
So I decided to try option 2, and compile it from source.

While there is a page in the docs on how to compile taichi for development purposes, with the accompanying scripts to go, I used the build.yaml workflow from GitHub to set the correct cmake flags and dove into the build scripts on my own.

I am not able to use the build.py script, since it accesses a read-only folder in the sandbox during build, so I opted for the automated cmake build of nix (if cmake is provided as a dependency, it will execute it automatically).

I was able to hack something together and package taichi, and it seems to be working with OpenGL, but Vulkan and CUDA is not working correctly.

I am not expecting the issue to be resolved by taichi-devs, but maybe give me a hint on how to compile the package correctly without the provided build.py. Maybe someone is using nix themselves and would be eager to collaborate on packaging it 😄

The current PR can be found here: NixOS/nixpkgs#370780
(The previous attempt with packaging from PyPI is here: NixOS/nixpkgs#367617)
When I am home, I can provide additional logs from compiling!