gnome3.webkitgtk: 2.20.0 -> 2.20.1 (#38824)

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/webkitgtk/versions.

These checks were done:

- built on NixOS
- ran ‘/nix/store/4iba6cp8lj5v53lzk6j3djrnf790byj4-webkitgtk-2.20.1/bin/WebKitWebDriver -h’ got 0 exit code
- ran ‘/nix/store/4iba6cp8lj5v53lzk6j3djrnf790byj4-webkitgtk-2.20.1/bin/WebKitWebDriver --help’ got 0 exit code
- found 2.20.1 with grep in /nix/store/4iba6cp8lj5v53lzk6j3djrnf790byj4-webkitgtk-2.20.1
- directory tree listing: https://gist.github.com/a60788ffc43ea1bedb466120462ff111