chore(nix): bump rust-overlay from 40b0a3a to c30ca20

Bumps [rust-overlay](https://github.com/oxalica/rust-overlay) from `40b0a3a` to `c30ca20`.
- [Commits](oxalica/rust-overlay@40b0a3a...c30ca20)

---
updated-dependencies:
- dependency-name: rust-overlay
  dependency-version: c30ca201c5093540cf792f6982f81ba1aa0f3514
  dependency-type: direct:production
...

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Author: dependabot[bot]