Spree::Variant.in_stock: Only show distinct variants

[mamhoff]

Summary

Without the .distinct added to this scope, it returns the same variant multiple times if there is stock in multiple stock locations.

Checklist

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• [x] I have written a thorough PR description.
• [x] I have kept my commits small and atomic.
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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.23%. Comparing base (8182cb2) to head (b2f8e4f). Report is 6 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5860 +/- ## ======================================= Coverage 89.23% 89.23% ======================================= Files 752 752 Lines 17510 17510 ======================================= Hits 15625 15625 Misses 1885 1885 ``` | [Flag](https://app.codecov.io/gh/solidusio/solidus/pull/5860/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=solidusio) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/solidusio/solidus/pull/5860/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=solidusio) | `89.23% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=solidusio#carryforward-flags-in-the-pull-request-comment) to find out more.

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