Pinning quimb in CI

[PietropaoloFrisoni]

Pinning quimb to version 1.8.4 in the workflow file since version 1.9.0 (released on 2024-11-19) broke our CI.

The default.tensor device will be updated as soon as possible to be compatible with the newest quimb version.

[github-actions[bot]]

Hello. You may have forgotten to update the changelog! Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov[bot]]

Codecov Report

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

Project coverage is 99.45%. Comparing base (061c855) to head (ac87e8a). Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #6612 +/- ## ======================================= Coverage 99.45% 99.45% ======================================= Files 450 450 Lines 42088 42088 ======================================= Hits 41857 41857 Misses 231 231 ```

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