CI: stop masking a failing notebook build (#76)

The notebook-build step ends with `mkdir -p` and `cp -u`, so the step's exit
code is `cp`'s, not `jb build`'s. `--keep-going` guarantees the .ipynb files
exist for `cp` to succeed on, so a failing strict build reported success.
`shell: bash -l {0}` is a custom shell spec, so GitHub injects no `-eo pipefail`
of its own.

This is the last live instance of the pattern across the five editions:
lecture-intro.zh-cn and lecture-python.zh-cn already carry the fix, and the .fa
and .fr editions are not affected because their builds are the last command in
the step.

The step's twin, "Build HTML", runs the same `-n -W --keep-going` unmasked and
is green, so this bounds what removing the mask can reveal to failures specific
to the jupyter custom builder.

Author: mmcky

.github/workflows/ci.yml

@@ -40,6 +40,12 @@ jobs:
       - name: Build Download Notebooks (sphinx-tojupyter)
         shell: bash -l {0}
         run: |
+          # `shell: bash -l {0}` is a custom shell spec, so GitHub does not inject
+          # `-eo pipefail` (it only does that for the bare `shell: bash` shorthand).
+          # Without this, a failing `jb build` would be masked by the trailing
+          # mkdir/cp, whose exit code becomes the step's — and `--keep-going`
+          # guarantees the .ipynb files exist for `cp` to succeed on.
+          set -eo pipefail
           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