Fix nix build

[link2xt]

There has been a recent update to crates.io that resulted in crates.io refusing downloads to nix downloader based on the user agent.

The solution is NixOS/nixpkgs#524985, but this requires updating nixpkgs and upgrading resulted in other required changes. Switched to master branch of nixpkgs because it is the only one including the fix atm: https://nixpk.gs/pr-tracker.html?pr=524985
naersk also needs a similar fix, I have switched to PR branch while it is not merged: nix-community/naersk#391

sphinx_rtd_theme has been renamed to sphinx-rtd-theme

Moving pthreads to buildInputs fixes this error:

   error: Refusing to evaluate package 'mingw_w64-pthreads-13.0.0' in /nix/store/f78lkqnk63pd0kf52zf2wcx35p1nnalr-source/pkgs/os-specific/windows/mingw-w64/headers.nix:35 because it is not available on the requested hostPlatform:
     hostPlatform.system = "x86_64-linux"
     package.meta.platforms = [
       "aarch64-windows"
       "x86_64-windows"
       "i686-windows"
     ]
     package.meta.badPlatforms = [ ]