tools: add _depot_tools to PATH (for V8 tests)

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.

PR-URL: https://github.com/nodejs/node/pull/38299


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>