core.py: use ChaNGa build UUID as build's directory

The ability to truly make use of any and all available options for a
package is what necessitated this change: while storing a build's files
in a directory based on SOME of the build's configuration is more
human-friendly, it seems to have led to behaviors where Chimi would
happily overwrite other builds with the same directory (i.e., name) but
different configurations.