Unmount everything under /mnt before zpool export, not just the chroot dirs

The previous fix (#42) only unmounted /mnt/{dev,proc,sys,run}, but disko
also mounts the ESP at /mnt/boot (modules/disko/baremetal.nix) -- another
nested mount blocking ZFS from unmounting its own root dataset at /mnt
the same way. Confirmed live: zpool export still failed with "cannot
unmount '/mnt': pool or dataset busy" after the chroot-only fix.

Replace the manual dev/proc/sys/run list with a single recursive
`umount -R /mnt`, which clears everything nested under /mnt -- current
and future mountpoints alike -- rather than needing to keep enumerating
whatever nixos-install/disko happen to leave mounted.

scripts/installer/auto-install.sh

@@ -180,19 +180,22 @@ nixos-install \
     # 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
+    # Anything still mounted under /mnt -- nixos-install's own leftover
+    # chroot bind mounts for running the target's activation script
+    # (/mnt/dev, /mnt/proc, /mnt/sys, /mnt/run), and disko's own /mnt/boot
+    # ESP mount (modules/disko/baremetal.nix) -- blocks ZFS from unmounting
+    # its root dataset at /mnt, the same way any nested mount blocks
+    # unmounting its parent. Confirmed live: zpool export failed with
+    # "cannot unmount '/mnt': pool or dataset busy" even after handling the
+    # chroot mounts alone, because /mnt/boot was still mounted too. Because
+    # of this script's `set -e`, that killed the script before it ever
+    # reached reboot, silently defeating the whole point of exporting first.
+    # Unmounting everything under /mnt up front (recursively, so nested
+    # mounts like /mnt/dev/pts come along for free) sidesteps needing to
+    # enumerate every mount disko/nixos-install might leave behind.
+    if mountpoint -q /mnt; then
+        umount -R /mnt
+    fi
 
     if [[ -n "$(zpool list -H -o name 2>/dev/null)" ]]; then
         echo "Exporting ZFS pool(s) before reboot..."