[fix] lxc.sh - SC2034: ubu2010_boilerplate appears unused.

  $ make test.sh
  In utils/lxc.sh line 42:
  ubu2010_boilerplate="$ubu1904_boilerplate"
  ^-----------------^ SC2034: ubu2010_boilerplate appears unused. Verify use (or export if used externally).

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>