Unmount nixos-install's leftover chroot bind mounts before zpool export

nixos-install bind-mounts /dev, /proc, /sys (and usually /run) into
/mnt to run the target's activation script in a chroot, and doesn't
unmount them again afterward. Left in place, those nested mounts made
ZFS refuse to unmount its own root dataset at /mnt: zpool export
failed with "cannot unmount '/mnt': pool or dataset busy", and because
of this script's set -e, that killed the script before it ever reached
reboot -- silently defeating the export-before-reboot fix from #40 on
every real run, which is why the ZFS-import stall kept recurring.

modules/platforms/baremetal.nix

@@ -25,4 +25,14 @@
     enable = true;
     enable32Bit = true;
   };
+
+  # The systemd-based initrd (default here since this host has a ZFS root --
+  # see modules/disko/baremetal.nix) locks the root account by default, so
+  # sulogin refuses to hand over a shell if something in the initrd (e.g.
+  # the ZFS pool import) fails and it drops to emergency mode -- confirmed
+  # live: it just loops re-entering the target instead of prompting. This
+  # only affects the pre-switch-root initrd shell, not the installed
+  # system's own login, and is worth the tradeoff on a box already reachable
+  # at the physical console.
+  boot.initrd.systemd.emergencyAccess = true;
 }

scripts/installer/auto-install.sh

@@ -179,6 +179,21 @@ nixos-install \
     # makes boot stall waiting on the ZFS import. Exporting here (a no-op
     # if the chosen host has no ZFS root, e.g. proxmox-*/linode-*) clears
     # that in-use state so the next import, from any hostid, succeeds.
+    #
+    # nixos-install bind-mounts /dev, /proc, /sys (and usually /run) into
+    # /mnt to run the target's activation script (switch-to-configuration
+    # boot) in a chroot, and doesn't unmount them again once it's done.
+    # Left in place, those nested mounts make ZFS refuse to unmount its own
+    # root dataset at /mnt -- confirmed live: zpool export failed with
+    # "cannot unmount '/mnt': pool or dataset busy", and because of this
+    # script's `set -e`, that killed the script before it ever reached
+    # reboot, silently defeating the whole point of exporting first.
+    for chroot_mount in dev proc sys run; do
+        if mountpoint -q "/mnt/${chroot_mount}"; then
+            umount -R "/mnt/${chroot_mount}"
+        fi
+    done
+
     if [[ -n "$(zpool list -H -o name 2>/dev/null)" ]]; then
         echo "Exporting ZFS pool(s) before reboot..."
         zpool export -a