[collab] Source pip dependencies from environment.yml

[mmcky]

What

The Colab execution workflow (collab.yml) duplicated the full pip dependency list as a literal pip install line. That copy drifted out of sync with environment.yml: Dependabot's conda ecosystem only updates environment.yml, so after #319 bumped quantecon-book-theme to 0.21.0 the workflow was still pinned to 0.20.2. This was surfaced (and is now superseded) by #303.

This change derives the pip dependencies from environment.yml at build time, giving a single source of truth so the two can no longer diverge:

python -c "import yaml; deps = next(d['pip'] for d in yaml.safe_load(open('environment.yml'))['dependencies'] if isinstance(d, dict) and 'pip' in d); open('colab-requirements.txt', 'w').write('\n'.join(deps) + '\n')"
cat colab-requirements.txt
pip install -r colab-requirements.txt

Why this approach

• No Dependabot config change needed — it keeps updating environment.yml as before, and the Colab workflow automatically picks up those updates.
• The container is pip-only (Google Colab runtime image), so we extract just the pip: subsection rather than recreating the conda env.
• The generated list is printed to the log (cat colab-requirements.txt) for visibility.

Verification

Validated locally that the snippet extracts the correct list (now quantecon-book-theme==0.21.0, matching environment.yml) and that collab.yml is well-formed YAML. The execution-checks job on this PR exercises the new install step end-to-end on GPU.

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Pull request overview

This PR updates the Google Colab execution workflow to derive its pip install requirements directly from environment.yml, eliminating drift between the workflow’s pinned dependencies and the repo’s single source of truth.

Changes:

• Generate colab-requirements.txt at runtime by extracting the pip: dependency list from environment.yml.
• Install Colab build dependencies via pip install -r colab-requirements.txt (and print the generated list to logs).

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