The multi-snapshot feature is only used on bots, where we need the
ability to execute tests with/without mitigations controllable through a
runtime flag.

This disables the flag by default and only enables it on bot configs.

Note: standard single-snapshot builds silently use the available
snapshot regardless of the value of --untrusted-code-mitigations.

Bug: v8:7891,v8:6688
Change-Id: I0d1a91e5c1c17b4d91b42d176b50ea44d2128d48
Reviewed-on: https://chromium-review.googlesource.com/1117687
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54079}