Use nightly tests in CI

[beorostica]

The idea is to regularly test with the Julia nightly version (the latest version under development) to avoid surprises when a new Julia version is released

[beorostica]

We aim to test both stable Julia and nightly Julia. However, if the nightly build fails while the stable build passes, we would like to generate a 'ticket' for the entire CI process and a 'warning' to indicate that the 'nightly' build is not successful.

Unfortunately, it seems that this feature is not currently implemented in GitHub Actions. You can refer to actions/runner#2347 for more information.

[beorostica]

For the time being, we have configured the following settings:

• We use the if: always() statement for stable tests.
• For nightly tests, we employ the continue-on-error: true statement.

This configuration allows for the scenario where the nightly tests consistently pass (it displays a 'ticket'), while the stable tests may either throw an error or pass. In both cases, all tests for the submodules are executed. It is crucial to regularly inspect the log messages of the nightly tests because the passed 'ticket' in this case doesn't provide much information.