fix: update notebooks to avoid using the nutsy --image-name __system__ thing (#1308)

The `--image-name __system__` thing was a hack and a bad one at that.
The actual intent was to somehow automatically detect the notebook
environment so we could avoid unnecessarily confusing things in the
llama stack build cmd-line. But I failed which led us to use the backup
`__system__` thing.

Let's just do the simple thing.

Note that `build_venv.sh` I haven't changed for now (so it still honors
the __system__ special name just that no new user should use it.)

## Test Plan

Open the notebooks from this branch in Colab (see example url below) and
ensure the builds work.


https://colab.research.google.com/github/meta-llama/llama-stack/blob/foo/docs/getting_started.ipynb

In the notebook, install llama-stack from this branch directly using:

```
!pip install -U https://github.com/meta-llama/llama-stack/archive/refs/heads/foo.zip
```

Verify that `!UV_SYSTEM_PYTHON=1 llama stack build --template together
--image-type venv` afterwards succeeds and the library client
initialization also works.

llama_stack/cli/stack/_build.py

@@ -38,7 +38,7 @@ from llama_stack.distribution.distribution import get_provider_registry
 from llama_stack.distribution.resolver import InvalidProviderError
 from llama_stack.distribution.utils.config_dirs import DISTRIBS_BASE_DIR
 from llama_stack.distribution.utils.dynamic import instantiate_class_type
-from llama_stack.distribution.utils.exec import formulate_run_args, in_notebook, run_with_pty
+from llama_stack.distribution.utils.exec import formulate_run_args, run_with_pty
 from llama_stack.distribution.utils.image_types import ImageType
 from llama_stack.providers.datatypes import Api
 
@@ -65,8 +65,6 @@ def run_stack_build_command(args: argparse.Namespace) -> None:
     if args.image_type == "venv":
         current_venv = os.environ.get("VIRTUAL_ENV")
         image_name = args.image_name or current_venv
-        if not image_name and in_notebook():
-            image_name = "__system__"
     elif args.image_type == "conda":
         current_conda_env = os.environ.get("CONDA_DEFAULT_ENV")
         image_name = args.image_name or current_conda_env
@@ -291,6 +289,8 @@ def _run_stack_build_command_from_build_config(
         if not image_name:
             raise ValueError("Please specify an image name when building a conda image")
     elif build_config.image_type == ImageType.venv.value:
+        if not image_name and os.environ.get("UV_SYSTEM_PYTHON"):
+            image_name = "__system__"
         if not image_name:
             raise ValueError("Please specify an image name when building a venv image")