This runs tests against Debian stable (bullseye) and, once rocksdb
6.25 makes it out of experimental, against bookworm too (testing).

Following suggestions from code review, this commit avoid re-using the working
directory from different versions of the library.

After many tries, I think I finally manage to find the right combination of
tweaks to make pytest, tox, and GitHub actions to work.

This also adds caching of the librocksdb compilation, which is a very slow
step, and restricts building of wheels and other artifacts to run only when
pushing to main.