Motivation for this change

The build sometimes fails, due to some necessary header files being generated too late. Building with --cores 1 seems to always trigger the issue. See upstream bug: https://bugs.webkit.org/show_bug.cgi?id=183788

This is a stopgap which simply generates the header files before compilation starts (at postConfigure) and should be removed when the problem is fixed upstream. It might not be ideal, but seems far better than having webkitgtk hold up nixos-unstable, and in the best case causing manual labor restarting hydra jobs.

fixes #37878

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.