No displayed output when embedding glued figures from another page/notebook

[markusritschel]

Describe the bug

When I want to embed glue output from other pages, it works with variables but not with figures.

Reproduce the bug

In my jupyter notebook examples/test.ipynb, I have created the following glue object:

# ... create some fig = plt.figure() object ...
from myst_nb import glue
my_variable = "here is some text!"
glue("my_text", my_variable, display=False)
glue("my_figure", fig, display=False)

Now, outside that notebook, in another markdown file, the directive

{glue}`examples/test.ipynb::my_text`

yields the expected output 'here is some text!'.

However, the directive

{glue}`examples/test.ipynb::my_figure`

only returns <Figure size 640x480 with 1 Axes>, without displaying the actual graphic. The following variants don't return anything at all:

{glue:figure}`examples/test.ipynb::my_figure`

```{glue} examples/test.ipynb::my_figure

### List your environment

I'm running the following configuration:
```shell
❯ jupyter-book --version                           
Jupyter Book      : 1.0.0
External ToC      : 1.0.1
MyST-Parser       : 2.0.0
MyST-NB           : 1.0.0
Sphinx Book Theme : 1.1.2
Jupyter-Cache     : 1.0.0
NbClient          : 0.10.0

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