Run sync workflow only when main branch is updated

[dhruvkb]

Description

This PR prevents the sync workflow from running except when main branch has changed. This means that the workflow will not run for other branches or PRs.

Technical details

See the discussion: WordPress/openverse-frontend#268 (comment)

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main or master).
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

Developer Certificate of Origin

Developer Certificate of Origin

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[zackkrida]

Thanks for the fix!