ci: really use shallow clones on Azure Pipelines

This was a left-over from the previous YAML schema, and it no longer
works. The problem was noticed while editing `azure-pipelines.yml` in VS
Code with the very helpful "Azure Pipelines" extension (syntax
highlighting and intellisense for `azure-pipelines.yml`...).

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

Author: dscho

azure-pipelines.yml

@@ -1,6 +1,5 @@
-resources:
-- repo: self
-  fetchDepth: 1
+variables:
+  Agent.Source.Git.ShallowFetchDepth: 1
 
 jobs:
 - job: windows_build