CI: cache tox environments by branch name

[sk1p]

If we change Python dependencies, to still be able to properly perform stable point releases, they should be independent from the tox environments that are cached for the master branch.

[sk1p]

It's possible to set the tox work dir via environment variable: toxworkdir={env:TOX_WORK_DIR:{toxinidir}/.tox} - if we then set TOX_WORK_DIR to something like tox-$System.PullRequest.TargetBranch in the pipeline, we should have different tox work dirs based on branch.