[FIX] Lift substitution of GH variables to sync.yml template to be propagated

[alyssadai]

Changes proposed in this pull request:

• Use of templates in repo-file-sync action doesn't play nicely with GH variable substitution present in the same workflow files, as this leads to conflicts in use of {{ }} for variable expansion
  • see https://github.com/neurobagel/bagel-cli/pull/271/files#diff-10b217627ccf6e5163f99ce9623fb2f56a3d3b4b1de4d022c864824f2d95e6ba for example
• To try and remedy this, this PR makes the GH variable substitution part of the template as well so the correct syntax is propagated to all workflow files for repo-level substitution

Checklist

• [x] PR has an interpretable title with a prefix ([ENH], [FIX], [REF], [TST], [CI], [MNT], [INF], [MODEL], [DOC]) _(see https://neurobagel.org/contributing/pull_requests for more info)_
• [x] 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.