[BUG] Fix GitHub repo variable in test workflow

[michellewang]

• Closes [BUG] ZENODO_ID not passed to API tests #701

Changes proposed in this pull request:

• Use vars instead of env context

Checklist (for reviewers)

This section is for the PR reviewer

• PR has an interpretable title with a prefix (e.g. [BUG], [DOC], [ENH], [MAINT])
  Refer to NumPy Development Guide for a full list
• PR links to GitHub issue with mention Closes #XXXX
• Tests pass
• Checks pass

For new features:

• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions

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📚 Documentation preview 📚: https://nipoppy--702.org.readthedocs.build/en/702/

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