fix(cli): Missing default for --image-type in stack run command (#1274)

# What does this PR do?

I think this got accidentally removed as part of
#1250. cc @leseb

## Test Plan

After the change, this arg is no longer required.

Signed-off-by: Yuan Tang <[email protected]>

llama_stack/cli/stack/run.py

@@ -73,6 +73,7 @@ def _add_arguments(self):
             type=str,
             help="Image Type used during the build. This can be either conda or container or venv.",
             choices=["conda", "container", "venv"],
+            default="conda",
         )
 
     def _run_stack_run_cmd(self, args: argparse.Namespace) -> None: