Fix running stack built with base conda environment (#903)

Fixes: #902

For the test verified that llama stack can run if built:
* With default "base" conda environment
* With new custom conda environment using `--image-name XXX` option
In both cases llama stack starts fine (was failing with "base") before
this patch.

CC: @ashwinb

Signed-off-by: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>

llama_stack/cli/stack/run.py

@@ -140,6 +140,10 @@ class StackRun(Subcommand):
                 return
 
             def get_conda_prefix(env_name):
+                # Conda "base" environment does not end with "base" in the
+                # prefix, so should be handled separately.
+                if env_name == "base":
+                    return os.environ.get("CONDA_PREFIX")
                 # Get conda environments info
                 conda_env_info = json.loads(
                     subprocess.check_output(