Substitution in Block doesn't seem to work in Live Preview

[salomon-chung]

What version of myst-parser are you using?

3.0.0

What version dependencies are you using?

pyodide-0.21.3

What operating system are you using?

Linux

Describe the Bug

Running the following code gives errors in https://myst-parser.readthedocs.io/en/latest/live-preview.html:

---
myst:
    substitutions:
        key: "I'm a **substitution**"
---
# Test

## Text

{{ key }}

## Code Block

{{
    "```\n" + key + "\n```"
}}

## Code Block + Environment

{{
    "```\n" + env.docname + "\n```"
}}

Expected Behavior

Substitution in block seems to work in this MWE:

• https://gitlab.com/uranie-cea/documentation/substitution-in-block/-/blob/main/test.md?ref_type=heads&plain=1
• https://substitution-in-block-uranie-cea-documentation-de220cba8224ddaf.gitlab.io/test.html

Is it the expected behaviour?

To Reproduce

1. Go to https://myst-parser.readthedocs.io/en/latest/live-preview.html
2. In Configuration (YAML), define:

   myst_enable_extensions:
   - substitution
   

3. Copy the previous code.

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[chrisjsewell]

Heya, as it mentions on the preview page, this is running the docutils parser, not the sphinx one, and so there is no env (i.e. BuildEnvironment) which a sphinx only thing