TST: CI should run one Azure Pipelines sdist build in PR #12271

To avoid finding out that Azure Pipelines sdist builds broken by new changes too late (again), there is a need to enable one representative sdist build using Azure Pipelines in a pull request. Unfortunately, this job cannot be skipped using [ci skip] directive in a commit message, so it will always run.

Blocked by:

• sdist & wheel builds broken #12170