Add run(s) against fresh build of git-annex

[yarikoptic]

It is a bit of a chicken/egg problem here - where to place testing of extensions against fresh build of git-annex -- in this repo or https://github.com/datalad/git-annex . I am leaning toward, add it here, probably not even across all OSes, e.g. just Linux, so we do not miss some regression git-annex might introduce within datalad extensions.

WDYT @jwodder -- here or git-annex?

[jwodder]

@yarikoptic Assuming the goal is for extension tests to be run every time git-annex is built, I think we should keep the extension tests here, because they're already here. We'd just need a way to trigger runs of the extension test workflow(s) from the build workflow, which can be done by adding a workflow_dispatch trigger to the former and adding a script (either a single curl command or a Python script, depending on how complex we need it to be) to the latter. The biggest caveat is that we may want to be able to prevent race conditions by passing the number of the build workflow to the extension test workflow in order to ensure that the correct triggering artifact gets downloaded; this would require changes to download-latest-artifact and/or datalad-installer.

[yarikoptic]

I think that may be we don't even need to get that "fancy", and on the CRON job as the tests are run here just run download-latest-artifact or datalad-installer to fetch the most recent version available, even if it might be a day old. Might be causing a bit of "heterogeneity" across extensions but IMHO it is ok. The main point is that the dashboard should stay green ;-)