fix: Use CONDA_DEFAULT_ENV presence as a flag to use conda mode (#1555)

# What does this PR do?

This is the second attempt to switch to system packages by default. Now
with a hack to detect conda environment - in which case conda image-type
is used.

Note: Conda will only be used when --image-name is unset *and*
CONDA_DEFAULT_ENV is set. This means that users without conda will
correctly fall back to using system packages when no --image-* arguments
are passed at all.

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan

Uses virtualenv:

```
$ llama stack build --template ollama --image-type venv
$ llama stack run --image-type venv ~/.llama/distributions/ollama/ollama-run.yaml
[...]
Using virtual environment: /home/ec2-user/src/llama-stack/schedule/.local
[...]
```

Uses system packages (virtualenv already initialized):

```
$ llama stack run ~/.llama/distributions/ollama/ollama-run.yaml
[...]
INFO     2025-03-27 20:46:22,882 llama_stack.cli.stack.run:142 server: No image type or image name provided. Assuming environment packages.
[...]
```

Attempt to run from environment packages without necessary packages
installed:
```
$ python -m venv barebones
$ . ./barebones/bin/activate
$ pip install -e . # to install llama command
$ llama stack run ~/.llama/distributions/ollama/ollama-run.yaml
[...]
ModuleNotFoundError: No module named 'fastapi'
```

^ failed as expected because the environment doesn't have necessary
packages installed.

Now install some packages in the new environment:

```
$ pip install fastapi opentelemetry-api opentelemetry-sdk opentelemetry-exporter-otlp aiosqlite ollama openai datasets faiss-cpu mcp autoevals
$ llama stack run ~/.llama/distributions/ollama/ollama-run.yaml
[...]
Uvicorn running on http://['::', '0.0.0.0']:8321 (Press CTRL+C to quit)
```

Now see if setting CONDA_DEFAULT_ENV will change what happens by
default:

```
$ export CONDA_DEFAULT_ENV=base
$ llama stack run ~/.llama/distributions/ollama/ollama-run.yaml
[...]
Using conda environment: base
Conda environment base does not exist.
[...]
```

---------

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

llama_stack/cli/stack/utils.py

@@ -4,6 +4,14 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+from enum import Enum
+
+
+class ImageType(Enum):
+    CONDA = "conda"
+    CONTAINER = "container"
+    VENV = "venv"
+
 
 def print_subcommand_description(parser, subparsers):
     """Print descriptions of subcommands."""