On some systems, chroot umounts blocked by "target busy"

On a completely fresh system:

# ./autobuilder
  * Checking for build-time dependencies...
  * Checking that the selected mode is valid...
  * Checking for a compatible host platform...
  * Checking that we are running with privileges...
  * Building a list of compatible architectures...
  * Building libcap2-2.62...
  * Building bubblewrap-798ced8a65c4e4d94ad8610d2d67376410eb6b54...
  * Preparing rootfs: aarch64...
  * Cleaning up any leftover mounts, if present...
  * Updating rootfs environment(s)...
fetch https://distfiles.adelielinux.org/adelie/1.0/system/aarch64/APKINDEX.tar.gz
fetch https://distfiles.adelielinux.org/adelie/1.0/user/aarch64/APKINDEX.tar.gz
system 1.0-RC2-156-g92bbe115 [https://distfiles.adelielinux.org/adelie/1.0/system]
user 1.0-RC2-160-ge455e006 [https://distfiles.adelielinux.org/adelie/1.0/user]
OK: 7702 distinct packages available
(1/5) Upgrading musl (1.2.0-r1 -> 1.2.0-r2)
(2/5) Upgrading musl-utils (1.2.0-r1 -> 1.2.0-r2)
(3/5) Upgrading ncurses-terminfo-base (6.2-r0 -> 6.2-r1)
(4/5) Upgrading ncurses-terminfo (6.2-r0 -> 6.2-r1)
(5/5) Upgrading ncurses-libs (6.2-r0 -> 6.2-r1)
OK: 38 MiB in 40 packages
umount: /.autobuilder/aarch64/rootfs/common: target is busy.
  * Cleaning up any leftover mounts, if present...
umount: /.autobuilder/aarch64/rootfs/packages: not mounted.