Update flake setup

[chrjabs]

Description of the Contribution

Cleans up the flake setup a bit, changes to rust-overlay, includes kani, and updates CaDiCaL in rustsat-tools derivation.

PR Checklist

• I read and agree to CONTRIBUTING.md
• I have formatted my code with rustfmt / cargo fmt --all
• Commits are named following conventional commits
• I have added documentation for new features
• The test suite still passes on this PR
• I have added tests for new features / tests that would have caught the bug this PR fixes (please explain if not)
• If this PR contains breaking changes, it is against the next-major branch, not against main