Set PATH more realistically in keystone_install_test.

Also adds comments to describe the relationship between the fake PATH
setup and the real one, in a way that presubmits can notice.

NO_IFTTT=new IFTTT region around existing code; this change fixes
preexisting divergence in keystone_install_test.sh, the source of
truth in install_from_archive.mm is fine

Fixed: 410898175
Change-Id: I8d372abd29f253a00baa06cef39410c89e5b13e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6460722
Reviewed-by: Noah Rose Ledesma <noahrose@google.com>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Adam Norberg <norberg@google.com>
Cr-Commit-Position: refs/heads/main@{#1447514}