Debian builds should use the packaged versions of the tools, only,
which seems achievable with pip's --no-build-isolation. This commits
also adds a verbose flag, to give the ability to easily verify this.

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).