The previous commit restricting build.yml to only run on 'main'
(same as newly-introduced debian.yml). But it's useful to be able
to see CI runs, during development _before_ opening a pull request.

A branch pattern is added so that forcing CI to run is possible.

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.