build,win: set env before generating projects

vcbuild.bat calls python configure before setting GYP_MSVS_VERSION,
so SelectVisualStudioVersion (tools\gyp\pylib\gyp\MSVSVersion.py)
defaults to 'auto' and selects VS 2005.

vcbuild sets the environment in the current shell, so this issue
would manifest itself only on the first invocation of the script
in any given shell windows.

Reviewed-By: Julien Gilli <jgilli@fastmail.fm>
PR-URL: https://github.com/joyent/node/pull/20109