ci: reinstate Azure Pipelines support

... so that we can test a MinGit backport in a private repository (with
GitHub Actions, minutes and parallel jobs are limited way more than with
Azure Pipelines in private repositories).

In this commit, we reinstate the exact version of `azure-pipelines.yml`
as 6081d38 (ci: retire the Azure Pipelines definition, 2020-04-11)
deleted.

Naturally, many adjustments are required to make it work again. Some of
the changes are actually outside of that file (such as the
`runs_on_pool` changes that are needed in the Azure Pipelines part of
`ci/lib.sh`) and they were made in the commits leading up to this here
commit.

However, other adjustments are required in the `azure-pipelines.yml`
file itself, and for ease of review (read: to build confidence in those
changes) they will be made in subsequent, individual commits that
explain the intent, context, implementation and justification like every
good commit message should do.

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