Motivation for this change

Update rtl8812au to latest version.

I noticed the previous version did not build with 4.15 kernels. The Grawp/rtl8812au_rtl8821au repo is no longer being maintained, so I've switched to the repo found from AUR: https://aur.archlinux.org/packages/rtl8812au-dkms-git/

Realtek's changelog: (which is where I got the more precise version string)
https://github.com/zebulon2/rtl8812au-driver-5.2.20/blob/master/Realtek_Changelog.txt
Note also that this latest version includes a fix for KRACK attacks.

I've tested this with my own TP-LINK AC1200 USB wifi adapter on 4.15 and it seems to be working fine. nox-review also ensures this builds against all the other kernel versions currently in nixpkgs.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.