tools: fix setting path containing an ampersand

This commit fixes an issue with the Node.js command prompt
on Windows where the PATH environment variable would not be
set correctly if the existing PATH contained an '&'.

Fixes: https://github.com/nodejs/node/issues/4802
PR-URL: https://github.com/nodejs/node/pull/4804


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>