feat: nix flake support

[hraban]

Hi all,

I created a flake to run this on NixOS. I'm sharing it here in the hopes it might be useful to someone else.

I won't sign the CLA though--my contribution is licensed under the GPL so you are free to include it in your project, but I'm not signing away copyright.

Regardless of that, it might be useful to someone some day, so even if it isn't merged in, here it is :)

If you want to try it out:

nix run github:trojan-gfw/trojan/pull/682/head -- --help

Thank you for your code, and best wishes 👍

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}