Mark pub's update_locked_test as flaky.

sdk/lib/_internal/pub/pub.status

Index: sdk/lib/_internal/pub/pub.status
diff --git a/sdk/lib/_internal/pub/pub.status b/sdk/lib/_internal/pub/pub.status
index c53b76301145f6ea12d7afa4a696e9bd3b610d47..1e72e02159b661c966f8a7a16cd011a78ff73d0e 100644
--- a/sdk/lib/_internal/pub/pub.status
+++ b/sdk/lib/_internal/pub/pub.status
@@ -21,6 +21,9 @@ test/install/hosted/unlock_if_incompatible_test: Pass, Fail # Issue 12837
 test/install/hosted/unlock_if_new_is_unsatisfied_test: Pass, Fail # Issue 12837
 test/install/hosted/stay_locked_test: Pass, Fail # Issue 12837
 
+[ $runtime == vm && $system == windows ]
+test/update/git/update_locked_test: Pass, Fail # Issue 12983
+
 # Pub only runs on the VM, so just rule out all compilers.
 [ $compiler == dart2js || $compiler == dart2dart ]
 *: Skip