fix: change downstream ci to use main branch

[Oisin-M]

Description

nit but noticed downstream-ci not using main

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.33%. Comparing base (47f963d) to head (96b272f).
⚠️ Report is 33 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (47f963d) and HEAD (96b272f). Click for more details.

HEAD has 3 uploads less than BASE

Flag	BASE (47f963d)	HEAD (96b272f)
	4	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #18       +/-   ##
===========================================
- Coverage   97.33%   63.33%   -34.01%     
===========================================
  Files           8        8               
  Lines         525      540       +15     
===========================================
- Hits          511      342      -169     
- Misses         14      198      +184     

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[awarde96]

Closing as this is merged into #20