How to install packages in chroot when chroot is launched with doas?

On OpenBSD 7.2 stable, I've setup my chroot using this script:

#!/bin/ksh

dir="/jails/$1"
mkdir -p "$dir"

tar -C "$dir" -xzf /usr/src/base/base72.tgz
tar -C "$dir" -xzf /usr/src/base/comp72.tgz
tar -C "$dir" -xzf /usr/src/base/man72.tgz
tar -C "$dir" -xzf /usr/src/base/xbase72.tgz
tar -C "$dir" -xzf /usr/src/base/xfont72.tgz
tar -C "$dir" -xzf /usr/src/base/xserv72.tgz
tar -C "$dir" -xzf /usr/src/base/xshare72.tgz

(cd "$dir/dev" && doas ./MAKEDEV all)

doas cp /etc/{doas.conf,installurl,resolv.conf,master.passwd,passwd,group} "$dir/etc/"
mkdir -p "$dir/home/evan"
chown evan:wheel "$dir/home/evan"

doas chroot "$dir" pwd_mkdb /etc/master.passwd
doas chroot "$dir" ldconfig /usr/local/lib
doas chroot "$dir" sysmerge

While logged-in as the user evan, I enter my chroot using doas chroot /jails/project /usr/bin/su - evan I try running doas pkg_add cowsay and I get this error:

doas: not installed setuid

What am I missing?