CI: stop masking a failing notebook build

[mmcky]

The notebook-build step reports success when the build fails.

Mechanism

The step runs three commands:

jb build lectures --path-output ./ --builder=custom --custom-builder=jupyter -n -W --keep-going
mkdir -p _build/html/_notebooks
cp -u _build/jupyter/*.ipynb _build/html/_notebooks

The step's exit code is cp's, not jb build's. --keep-going guarantees the .ipynb files exist for cp to succeed on even when the build failed, so a failing strict build reports green. shell: bash -l {0} is a custom shell spec, so GitHub injects no -eo pipefail of its own — it only does that for the bare shell: bash shorthand.

Scope

This is the last live instance across the five editions. lecture-intro.zh-cn and lecture-python.zh-cn already carry the fix; the .fa and .fr editions are not affected because their builds are the last command in the step. The added comment matches the wording already used in lecture-python.zh-cn so the reason travels with the line.

Risk

Bounded. This step's twin, Build HTML, runs the same -n -W --keep-going unmasked — it is the last command in its own step — and is green on main. So removing the mask can only reveal failures specific to the jupyter custom builder, not a long-broken build in general. If CI does go red here, that is a real finding and worth triaging on its own rather than reverting.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes a GitHub Actions CI false-positive in the “Build Download Notebooks (sphinx-tojupyter)” step by ensuring the step fails when jb build fails, rather than accidentally reporting success based on the trailing cp command’s exit status.

Changes:

• Add an explanatory comment describing why shell: bash -l {0} needs explicit error-handling flags.
• Add set -eo pipefail so the step stops on failures and propagates non-zero exit codes correctly.

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