Azure: Ensure we do a shallow git clone

In Azure, older pipelines such as ours do not default to a shallow fetch
but rather do a complete clone. This introduces a marginal time increase
in each task, but also more importantly takes up significant disk space.
We are now getting warnings in some cases about using more than 95% of
our available disk space so take this as a first easy step to resolve
that problem.

Signed-off-by: Tom Rini <trini@konsulko.com>

.azure-pipelines.yml

@@ -3,6 +3,8 @@ variables:
   ubuntu_vm: ubuntu-24.04
   macos_vm: macOS-14
   ci_runner_image: trini/u-boot-gitlab-ci-runner:jammy-20251001-04Nov2025
+  # Ensure we do a shallow clone
+  Agent.Source.Git.ShallowFetchDepth: 1
   # Add '-u 0' options for Azure pipelines, otherwise we get "permission
   # denied" error when it tries to "useradd -m -u 1001 vsts_azpcontainer",
   # since our $(ci_runner_image) user is not root.