add test for `llama stack build --run`

[mattf]

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🚀 Describe the new functionality needed

```llama stack build ... --run` is a valuable feature with no test coverage

💡 Why is this needed? What if we don't build it?

the feature may break, see https://github.com/meta-llama/llama-stack/issues/2188

Other thoughts

consider collapsing the Set Up Environment and Install Dependencies and Start Llama Stack server in background steps into one w/ --run

see https://github.com/meta-llama/llama-stack/blob/main/.github/workflows/integration-tests.yml#L44-L61