nixos-install: fix removal of non-empty /mnt

After building the target system, `nixos-install` tries to remove `/mnt`
on the target filesystem. And the script may fail without any explanation,
if `/mnt` isn't empty.

This commit makes the installation process carry on even if there are
files under `/mnt`.

See https://github.com/NixOS/nixpkgs/issues/244643.

nixos/modules/installer/tools/nixos-install.sh

@@ -206,7 +206,7 @@ if [[ -z $noBootLoader ]]; then
       mount --rbind --mkdir / "$mountPoint"
       mount --make-rslave "$mountPoint"
       /run/current-system/bin/switch-to-configuration boot
-      umount -R "$mountPoint" && rmdir "$mountPoint"
+      umount -R "$mountPoint" && (rmdir "$mountPoint" 2>/dev/null || true)
 EOF
 )"
 fi