rescue/sh check failed, installation aborted

From: Graham Perrin <grahamperrin_at_gmail.com>
Date: Mon, 23 Aug 2021 19:18:49 UTC
Encountered whilst attempting to build and install 14.0-CURRENT over 
13.0-RELEASE-p3 (experimental, helloSystem):

<https://i.imgur.com/euFBA8M.png>

Background, condensed, to the best of my recollection:

cd /usr/src
make buildworld    # succeeded
make kernel    # failed
make clean
LOCAL_MODULES=    # added to /etc/src.conf
make kernel-toolchain
make kernel
restarted in single user mode
mount -uw /
service zfs start
cd /usr/src
make installworld – failed as pictured.

I see <https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=231325>, fixed 
in 2018.

Any suggestions?

TIA