Move RunGN lower so that modifications to os.environ take effect

Without this the os.environ changes for various default values are
not taken into effect by GN.

R=brettw@chromium.org
BUG=323300

Review URL: https://codereview.chromium.org/139143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244963 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244963