Fix build recipe execution and flatten reliability

- Set HOME in bctx.EnvVars when USER switches so ~ expands correctly in
  subsequent RUN/WORKDIR steps instead of resolving to /root
- Run /bin/sync inside the guest before FlattenRootfs destroys the VM,
  preventing pip-installed files from being captured as 0-byte due to
  unflushed page cache
- Wrap healthcheck command with su <user> so it runs with the template's
  default user context (correct HOME, correct UID)
- Export Shellescape from the recipe package for use in build service
- Add code-runner-beta recipe (Jupyter server with ipykernel --sys-prefix)
  and replace old python-interpreter-v0-beta

recipes/code-runner-beta.recipefile

@@ -0,0 +1,9 @@
+RUN --timeout=5m sudo apt install -y python3 python3-pip python3-venv
+ENV PYTHONUNBUFFERED=1
+
+RUN python3 -m venv ~/jupyter-env
+RUN --timeout=5m ~/jupyter-env/bin/pip install --upgrade pip
+RUN --timeout=5m ~/jupyter-env/bin/pip install jupyter-server ipykernel
+RUN --timeout=5m ~/jupyter-env/bin/python -m ipykernel install --sys-prefix
+
+START ~/jupyter-env/bin/jupyter server --ServerApp.ip=0.0.0.0 --ServerApp.port=8888 --ServerApp.token='' --ServerApp.password='' --ServerApp.allow_origin='*' --ServerApp.disable_check_xsrf=True --no-browser --log-level=INFO