Introduce "downstream testing" workflow against keyrings.alt

[yarikoptic]

In reaction to #593 - it would be neat if there was a CI workflow which would do downstream testing against keyrings.alt released version. We do similar downstream testing e.g. in DataLad (https://github.com/datalad/datalad/blob/master/.github/workflows/test_extensions.yml) and other projects (e.g. for git-annex itself - testing against datalad in https://github.com/datalad/git-annex/) which helps tremendously to keep "ecosystem" healthy. Such testing would help to avoid such issues as #593 which might cause lots of ripple effects through downstream projects.