tasks: Drop obsolete environment variables
tasks: Drop obsolete environment variables

 - `T*MPDIR` is just the canonical default anyway
 - `TEST_PUBLISH` was empty, pointless
 - `LIBGUESTFS_BACKEND` isn't being used, libguestfs does not work in
   unprivileged containers. Also, if it's ever necessary, it should be
   done by bots' image-create.

cockpit-project/cockpituous,cockpit-project/cockpituous