The existing callsites of this are via `source` or being inline'd into
the startup of a new host; in both of these cases, the surrounding
script is already `set -eu`. However, if run as a standalone tool, it
should also configure itself to catch checksum failures and other
problems.
Even though this looks like an independently runnable script, it
should not be run independently: a SHA-256 mismatch will fail to stop
the script, unless it was sourced from another script that has ‘set
-e’.
Signed-off-by: Anders Kaseorg <anders@zulip.com>