Fix launch of system SBCL
Fix launch of system SBCL

Do not set SBCL_HOME to a bogus directory when the selected version is
the system one, let SBCL use its default, allowing it to find the
proper "sbcl.core".

This change makes 'ros run' work after 'ros use sbcl-bin/system'.

roswell/roswell,snmsts/roswell,roswell/roswell,snmsts/roswell,snmsts/roswell,roswell/roswell,roswell/roswell,snmsts/roswell