chore(nix): bump rust-overlay from 40b0a3a to 7cc96a6

Bumps [rust-overlay](https://github.com/oxalica/rust-overlay) from `40b0a3a` to `7cc96a6`.
- [Commits](oxalica/rust-overlay@40b0a3a...7cc96a6)

---
updated-dependencies:
- dependency-name: rust-overlay
  dependency-version: 7cc96a6a3fd6613cafd633250a3934483479b9a1
  dependency-type: direct:production
...

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Author: dependabot[bot]