nixos-install: add root directory ownership check

Prevents installation when mount point is not owned by root:root, which can cause systemd warnings and session failures.

Fixes #432261

pkgs/by-name/ni/nixos-install/nixos-install.sh

···
     checkPath="$(dirname "$checkPath")"
 done
 
+# Check if the mount point root directory is owned by root
+if [[ "$(stat -c '%u:%g' "$mountPoint")" != "0:0" ]]; then
+    echo "$0: $mountPoint is not owned by root:root (owned by $(stat -c '%U:%G' "$mountPoint"))"
+    echo "$0: installation root directory must be owned by root to avoid system issues"
+    echo "$0: run 'sudo chown root:root $mountPoint' to fix this"
+    exit 1
+fi
+
 # Verify that user is not trying to use attribute building and flake
 # at the same time
 if [[ -z $buildingAttribute && -n $flake ]]; then