[gn] Pass make path explicitly

We've been assuming make is in the user's path for generated Zircon
build rules. This assumption is false for Chromium bots, which are not
provisioned with build-essential and thus this is blocking Tricium
rollout, since that runs on Chromium bots.

Change-Id: I16cc31a282c4973c52012a1200976af3fe1d904a
3 files changed