fix insecure use of /tmp in EDSP solver 'dump'

fix insecure use of /tmp in EDSP solver 'dump'

As said in the bugreport, this is hardly a serious problem on a security
front, but it was always on the list to have the filename configurable
somehow and the stable filename is a problem for parallel executions.

Using an environment variable (APT_EDSP_DUMP_FILENAME) for this is more
or less the best we can do here as solvers do not get told about our
configuration and such.

Closes: 795600