[utils] Fix submodule logic for update-checkout script

Author: shahmishal

utils/update-checkout

@@ -96,7 +96,10 @@ def obtain_additional_swift_sources(
                     shell.call(['git', '--git-dir', src_path, '--work-tree',
                                 os.path.join(SWIFT_SOURCE_ROOT, dir_name),
                                 'checkout', repo_branch], echo=False)
-
+        with shell.pushd(os.path.join(SWIFT_SOURCE_ROOT, dir_name),
+                         dry_run=False, echo=False):
+            shell.call(["git", "submodule", "update", "--recursive"],
+                        echo=False)
 
 def validate_config(config):
     # Make sure that our branch-names are unique.