LLAMA_STACK_BASE_URL env no longer works

[mattf]

env LLAMA_STACK_BASE_URL was a valid way to specify the base_url.

please restore the old behavior.

it looks like 7ade969#diff-6368bdcbebce99630fe7471a8116c0a10186d1e8d0e162bed075541cac34beb5 is the change.

[mattf]

cc @ashwinb