Fix collection in case of derivatives with T1w and T2w but T1w is primary

[tsalo]

Closes #1239.

Changes proposed in this pull request

• If there's both a T1w and a T2w, but there's no T2w-to-template transform, use the T1w as the primary modality.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.

Project coverage is 84.04%. Comparing base (fec6022) to head (29a09bf). Report is 2 commits behind head on main.

Files	Patch %	Lines
xcp_d/utils/bids.py	0.00%	15 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1240 +/- ## ========================================== - Coverage 84.16% 84.04% -0.12% ========================================== Files 59 59 Lines 7172 7182 +10 Branches 1047 1048 +1 ========================================== Hits 6036 6036 - Misses 872 882 +10 Partials 264 264 ```

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