Create a workflow to test PennyLane with NumPy 1.26

[PietropaoloFrisoni]

Context: In #6061, we started using by default NumPy 2 (instead of NumPy 1) on the CI to test every PR targeting the master branch. Some jobs (for example, those requiring tensorflow) still automatically downgrade to NumPy 1.26 since not all interfaces are supported at the current stage.

This PR aims to create another workflow to test some jobs with NumPy 1.26 (those that don't automatically downgrade to NumPy 1 during the day). Such a workflow should run at night to ensure PennyLane is still compatible with NumPy 1.26.

We'll keep this workflow only temporarily (probably a quarter, but this still has to be established). After that, we'll get rid of it.

Finally, It has been verified (see here) that the tests introduced with this PR are green. This has been verified a second time (see here before merging this PR into master).

Description of the Change: As above.

Benefits: We ensure that PennyLane is still compatible with NumPy 1.26.

Possible Drawbacks:

• This issue was discovered accidentally (and potentially fixed) during this PR. The TestSinglePrecision class in the test_jax_jit_qnode.py file contains tests that change the JAX precision with jax.config.update("jax_enable_x64", False). When running the jax tests using multiple workers, we can face race conditions if some workers change jax configuration, because other workers in other test classes downgrade to a single precision causing numerous failures. To avoid this, we add --dist=loadscope as additional pytest arg for jax tests, grouping tests by module for test functions and by class for test methods.

• The new workflow is it is to a large extent a (very simplified) copy of interface-unit-tests.yml. This is against the principle of avoiding code duplication whenever possible. Still, this is just a temporary workflow that lasts for a few months at most (and runs only at night), and it may be convenient to disentangle it as much as possible from the usual test interface.

Related GitHub Issues: None.

Related Shortcut Stories: [sc-73744]

[github-actions[bot]]

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

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[codecov[bot]]

Codecov Report

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

Project coverage is 99.70%. Comparing base (de54c54) to head (7691a95). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #6275 +/- ## ======================================= Coverage 99.70% 99.70% ======================================= Files 444 444 Lines 42133 42133 ======================================= Hits 42008 42008 Misses 125 125 ```

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[PietropaoloFrisoni]

@rashidnhm Please ignore the failures related to JAX 0.4.28 and NumPy 1.26.4 (we'll decide what to do with them at a later stage). I asked for your review mostly for the workflow structure. Thanks!