qt5.qtwebengine: fix source exceeding output limit
qt5.qtwebengine: fix source exceeding output limit

This hack reduces the source derivation size from 3.7G to 450M by
removing the src/3rparty submodule .git directory and compressing
the whole directory. This is needed to avoid the 2G limit on hydra.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs