Update LKJ nb to avoid crashing at model seeding

[erik-werner]

Issue #53

• Update to do random seeding in the sample function instead of model building (see #5787)
• Remove unnecessary import and env variable

• Notebook follows style guide https://docs.pymc.io/en/latest/contributing/jupyter_style.html
• PR description contains a link to the relevant issue:
  • a tracker one for existing notebooks (tracker issues have the "tracker id" label)
  • or a proposal one for new notebooks
• Check the notebook is not excluded from any pre-commit check: https://github.com/pymc-devs/pymc-examples/blob/main/.pre-commit-config.yaml

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