Future improvement: detect existing mounts
While debugging #1 I forgot to umount the leftovers manually, and it went ahead and mounted over it again:
gwyn ~/autobuilder # mount | grep .autobuilder
/dev/nvme0n1p1 on /.autobuilder/ppc/rootfs/packages type ext4 (rw,relatime)
/dev/nvme0n1p1 on /.autobuilder/ppc/rootfs/var/cache/distfiles type ext4 (rw,relatime)
/dev/nvme0n1p1 on /.autobuilder/ppc/rootfs/packages type ext4 (rw,relatime)
/dev/nvme0n1p1 on /.autobuilder/ppc/rootfs/var/cache/distfiles type ext4 (rw,relatime)