Don't do build attestations on forks

[maresb]

Otherwise it fails with:

Error: Failed to get ID token: Error message: Unable to get ACTIONS_ID_TOKEN_REQUEST_URL env variable

Ugh, I meant to open this from my fork. 🤦‍♂️

Description

Related Issue

• Closes #
• Related to #

Checklist

• 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
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

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📚 Documentation preview 📚: https://pytensor--1167.org.readthedocs.build/en/1167/

[maresb]

Ugh, I meant to open this from my fork. 🤦‍♂️