Adding builds for Mercurial default and stable
The `--cleanup` flag is crucial here: because the kaniko builds happen sequentially, without the flag, the last one (fixed, released hg version) would have the Mercurial install shadowed by previous builds, resulting in a missing Mercurial. We've considered several options there, ideally we could have built directly from the Octobus ci-py3 image, but we lack a clear binary distribution format (there's `setup.py bdist` that produces a tarball rooted at /usr/local, but then we'd have to painfully extract and be 100% sure there's nothing to be cleaned up). Another option would have been simply pip install -r https://www.mercurial-scm.org/repo/hg/archive/default.tar.bz2 But even at a daily build rate, it's not really nice to the upstream server, and we could need punctiallly to produce several images in a row, as I am actually doing while preparing this. On the other hand, the Octobus ci-py3 image has clones attached to mirror.octobus.net, so that's the compromise we chose. In the future, we can also perhaps trigger from a periodic pull in https://foss.heptapod.net/octobus/mercurial-devel or in the pull from mercurial-mirror, building only for non trivial changes.