win: backport set env before generating projects

Backport commit a58b1745

 from branch
v0.12.

Original commit message:

  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



Conflicts:
	vcbuild.bat

Reviewed-By: João Reis <reis@janeasystems.com>
PR-URL: https://github.com/joyent/node/pull/25569