ci: Also build Rust nightly in Release mode

[def-]

Would have caught https://materializeinc.slack.com/archives/C08ACQNGSQK/p1759754676092499

Let's see if it's fixed in Nightly already: https://buildkite.com/materialize/nightly/builds/13849

Edit: Blocked on https://github.com/rust-lang/rust/issues/146998

Checklist

• [ ] This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• [ ] This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• [ ] If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• [ ] If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• [ ] If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[def-]

It fails: (signal: 9, SIGKILL: kill)

Locally it fails for me the same way as in https://materializeinc.slack.com/archives/C08ACQNGSQK/p1759754676092499, but a bit weird that it shows up as a signal 9 in CI without logging.