i've found exactly the same problem...
The file scan engine also use the /tmp folder located in the SSD for doing the scan.
That's not a problem, this is the intended behavior. The engine uses the /tmp directory for
extracted files, when scanning archives.
Again, when you are so concerned about temporary files written to your hard disk, then
use a RAM disk for the /tmp directory. This will solve your "problem" with not only Avast
but almost all SW on your Mac. You must be however prepared for some corner cases,
when the temporary files won't fit into the RAM disk.