<tools/gyp> Fix ninja_use_custom_environment_files

We want to use this option to prevent gyp from detecting the MSVC
toolchain by itself. We already require the environment to be setup
according to the desired toolchain for Qt, and we should respect it
when it's time to build QtWebEngine through gyp.

win_tool.py currently expects the environment.<arch> file to be
present. Fix the issue by copying in this file the whole environment
at the gyp invocation time instead of from an invocation of the
setup script when ninja_use_custom_environment_files isn't used.

Change-Id: Iade4c488e1af5cab8306e9070c73d42ec67a955a
Reviewed-by: Andras Becsi <andras.becsi@digia.com>