php: override outputsToInstall when using withExtensions
php: override outputsToInstall when using withExtensions

At the moment, using .withExtensions on a PHP derivation will
produce something which can't be used inside an
environment.systemPackages array, because outputsToInstall refers
to an output which doesn't exist on the final derivation.

Instead, override it back to just containing the single output
"out".

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