sbox: report the script path instead of the full command line

The command line run inside the sbox sandbox can be very long and
printing it obscures the error message.  It is already written to
a script for execution, so point to the script instead of printing
it.

Fixes: 220592170
Test: manual
Change-Id: Icf883344af0f707802b090b4cfb0ef6dcfe0e5c1