l: avoid "invalid subscript" error with zsh

* scripts/l: Use ${ESC}[ rather than $ESC[0
to avoid "invalid subscript" error with zsh.
Note `ksh -n` shows this apparent ambiguity well:
  bin/l: warning: line 183:
  use braces to avoid ambiguities with $id[...]
Note however that ksh, bash and dash were all fine
with the existing syntax.