18.104.22.168: Fix run-sbcl.sh to use absolute paths when necessary.
* The new ASDF 2 requires absolute paths in places where SBCL
historically used relative paths. The last(?) remaining place
where this happens is the SBCL_HOME environment variable set up
by run-sbcl.sh. Fixed by introducing a readlink -f to generate
an absolute path to the base SBCL directory.