Expose LLAMASTACK_PORT in cli.stack.run (#722)

This was missed in https://github.com/meta-llama/llama-stack/pull/706. I
tested `llama_stack.distribution.server.server` but didn't test `llama
stack run`. cc @ashwinb

Signed-off-by: Yuan Tang <terrytangyuan@gmail.com>

llama_stack/cli/stack/run.py

@@ -5,6 +5,7 @@
 # the root directory of this source tree.
 
 import argparse
+import os
 from pathlib import Path
 
 from llama_stack.cli.subcommand import Subcommand
@@ -34,7 +35,7 @@ class StackRun(Subcommand):
             "--port",
             type=int,
             help="Port to run the server on. Defaults to 5000",
-            default=5000,
+            default=int(os.getenv("LLAMASTACK_PORT", 5000)),
         )
         self.parser.add_argument(
             "--disable-ipv6",