Set multiprocessing to spawn for Linux

[fonnesbeck]

Description

Multiprocess sampling often fails randomly on Linux due to the start method defaulting to "fork". This sets it to "spawn" when mcmc is imported.

Related Issue

• [x] Closes #7354
• [ ] Related to #

Checklist

• [x] Checked that the pre-commit linting/style checks pass
• [ ] Included tests that prove the fix is effective or that the new feature works
• [ ] Added necessary documentation (docstrings and/or example notebooks)
• [ ] If you are a pro: each commit corresponds to a relevant logical change

Type of change

• [ ] New feature / enhancement
• [x] Bug fix
• [ ] Documentation
• [ ] Maintenance
• [ ] Other (please specify):

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📚 Documentation preview 📚: https://pymc--7804.org.readthedocs.build/en/7804/