fixup! ci: reinstate Azure Pipelines support

Trying to reinstate support for Azure Pipelines is somewhere between
heroic and futile. I originally tried to do this because it was highly
valuable to be able to run Git's test suite in a private Azure DevOps
project when developing embargoed releases.

Now that upstream drops even more Azure Pipeline support code, it is
time to declare defeat.

Signed-off-by: Johannes Schindelin <[email protected]>