ninja win: Allow parent environment variables through when running actions

This is a hack to get variables set here:
https://code.google.com/p/chromium/codesearch#chromium/tools/build/scripts/slave/compile.py&l=416
through to version.py

In preference to https://codereview.chromium.org/136163002/ as it doesn't
involve trying to modify where the variables are set in master build
scripts.

TBR=thakis@chromium.org
BUG=333738

Review URL: https://codereview.chromium.org/133503003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1836 78cadc50-ecff-11dd-a971-7dbc132099af