Incorrect result generated from sample of Jupyter Book

[zouguangxian]

Describe the problem

expect code in code-cell will be executed and output of result will be included in the generated file, but empty is placed at that place instead.

jb create sample
cd sample
printf '```{code-cell}\nprint("Here is some code to execute")\n```\n'
jb build --all ./

expect "Here is some code to execute" will be included at the end of _build/html/markdown.html.

Link to your repository or website

No response

Steps to reproduce

jb create sample
cd sample
printf '```{code-cell}\nprint("Here is some code to execute")\n```\n'
jb build --all ./

generated content is in _build/html/markdown.html.

The version of Python you're using

3.8.10

Your operating system

Linux 2549a707a148 5.10.47-linuxkit #1 SMP Sat Jul 3 21:51:47 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

Versions of your packages

# jupyter-book --version
Jupyter Book      : 0.12.1
External ToC      : 0.2.3
MyST-Parser       : 0.15.2
MyST-NB           : 0.13.1
Sphinx Book Theme : 0.1.7
Jupyter-Cache     : 0.4.3
NbClient          : 0.5.9

Additional context

No response

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