chore(nix): bump rust-overlay from 40b0a3a to 4a408e1

Bumps [rust-overlay](https://github.com/oxalica/rust-overlay) from `40b0a3a` to `4a408e1`.
- [Commits](oxalica/rust-overlay@40b0a3a...4a408e1)

---
updated-dependencies:
- dependency-name: rust-overlay
  dependency-version: 4a408e1fc99ad517b4cb402fd0de7464f40c05e1
  dependency-type: direct:production
...

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Author: dependabot[bot]