fix(ci): correct remaining path filters and Docker build args

Fixed path filters that still referenced the old llama_stack/ location:
- Updated negative path exclusions from !llama_stack/ui/** to !src/llama_stack/ui/**
- Updated Docker RUN_CONFIG_PATH build args to use /workspace/src/llama_stack/distributions/

These paths were missed in the initial src/ migration commit.

.github/workflows/test-external.yml

@@ -9,7 +9,7 @@ on:
     branches: [ main ]
     paths:
       - 'src/llama_stack/**'
-      - '!llama_stack/ui/**'
+      - '!src/llama_stack/ui/**'
       - 'tests/integration/**'
       - 'uv.lock'
       - 'pyproject.toml'