Default SBCL_HOME on Win32 is less than excellent

public bug reported:

SBCL_HOME on Win32 defaults to /usr/local/lib/sbcl/, which is less than excellent. The installer tries to set it to the directory that SBCL is installed into, but this only happens for the user running the installer. If another user tries to run SBCL and doesn't set SBCL_HOME first, the core file is not found.

As the installer stores the SBCL executable in SBCL_HOME, a better default would be the path to the executable.