Ensures the `_depot_tools` dir bootstrapped during this script
is added to the PATH, before `ninja` is executed,
as `_depot_tools` already has a copy of `ninja` in it.
This means one less system requirement to be able to run this script.

(It seems like this was already intended to be on the PATH?)

Note: This script is used by the `test-v8` Makefile target.

Simplifies the script... Also finds _depot_tools relative to
the current path, rather than relative to the repo root.

This seems like a good idea, given that the rest of the script is also
relative to the current path (`deps/v8`), not the repo root.