CI: Start validating on CrateDB nightly

## Problem
It is possible that changes to CrateDB will not be detected here, because the test suite does not employ nightly runs, and also does not test against CrateDB nightly. The effect is that only downstream tests will catch regressions. While this works well, it does not provide a dedicated signal for the [build status](https://cratedb.com/docs/guide/integrate/status.html) page.

## References
- https://github.com/crate/crate-jdbc/pull/475
- https://github.com/crate/crate-alerts/issues/864#issuecomment-4016317883

Start by reading the referenced crate-jdbc pull request #475 and the CrateDB alerts discussion in issue #864. Identify the existing CI validation entry point and how CrateDB versions are selected; done means nightly runs validate against CrateDB nightly and provide the dedicated build-status signal described in the issue.

Written by the indexing model from the issue text.