chore(nix): bump rust-overlay from 47ab6c7 to 6cddd51

Bumps [rust-overlay](https://github.com/oxalica/rust-overlay) from `47ab6c7` to `6cddd51`.
- [Commits](oxalica/rust-overlay@47ab6c7...6cddd51)

---
updated-dependencies:
- dependency-name: rust-overlay
  dependency-version: 6cddd512fa2bf7231f098d3a2f92f6e4cff71e0a
  dependency-type: direct:production
...

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Author: dependabot[bot]