gnome3.webkitgtk: 2.20.1 -> 2.20.2

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/h4aballc6wsb4kj32kcjcx5x4vbax6fj-webkitgtk-2.20.2/bin/WebKitWebDriver -h’ got 0 exit code
- ran ‘/nix/store/h4aballc6wsb4kj32kcjcx5x4vbax6fj-webkitgtk-2.20.2/bin/WebKitWebDriver --help’ got 0 exit code
- found 2.20.2 with grep in /nix/store/h4aballc6wsb4kj32kcjcx5x4vbax6fj-webkitgtk-2.20.2
- directory tree listing: https://gist.github.com/e3496e2d8a6b933a107141f5caecb92d