show_snapshot.py unpickled a user-supplied path unconditionally. pickle.loads
is equivalent to arbitrary code execution, so a snapshot from an untrusted
source = RCE. Require an explicit --i-trust-this-file acknowledgement before
calling pickle.loads, and emit a stderr warning when proceeding.
Co-authored-by: Jiahui-Gu <jiahuigu@users.noreply.github.com>