The check read /proc/$$/cmdline. arch-chroot runs its command in a new PID
namespace beneath the outer /proc, where $$ names no process or someone
else's, so every bash -p entrypoint was rejected there with status 126.
The installer met this as soon as one was called at install time:
install/config/thunderbolt-authorization.sh runs
omarchy-thunderbolt-authorization-admin, and the edge ISO stopped in
"Configuring system" with "/proc/219/cmdline: No such file or directory".
The check runs in a child of the caller, so read the parent from that
child's /proc/self/stat, which is given in the numbering of the /proc it is
read through. Outside a PID namespace that is $$, as before.
Emir Beganovic