fix: use OLLAMA_URL to activate Ollama provider in starter (#2963)

We tried to always keep Ollama enabled. However doing so makes the
provider implementation half-assed -- should it error when it cannot
connect to Ollama or not? What happens during periodic model refresh?
Etc. Instead do the same thing we do for vLLM -- use the `OLLAMA_URL` to
conditionally enable the provider.

## Test Plan

Run `uv run llama stack build --template starter --image-type venv
--run` with and without `OLLAMA_URL` set. Verify using
`llama-stack-client provider list` that ollama is correctly enabled.

docs/quick_start.ipynb

@@ -150,7 +150,7 @@
         "def run_llama_stack_server_background():\n",
         "    log_file = open(\"llama_stack_server.log\", \"w\")\n",
         "    process = subprocess.Popen(\n",
-        "        f\"uv run --with llama-stack llama stack run starter --image-type venv --env INFERENCE_MODEL=llama3.2:3b\",\n",
+        "        f\"OLLAMA_URL=http://localhost:11434 uv run --with llama-stack llama stack run starter --image-type venv",
         "        shell=True,\n",
         "        stdout=log_file,\n",
         "        stderr=log_file,\n",