tools: fix run-valgrind.py script

The script had a dependency on the copy of valgrind that is bundled
with V8 but that only gets checked out when doing a full depot_tools
checkout.  Use the system-provided valgrind.

PR-URL: https://github.com/nodejs/node/pull/9520


Reviewed By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>