chore(nix): bump rust-overlay from 40b0a3a to 7a5a1c0

Bumps [rust-overlay](https://github.com/oxalica/rust-overlay) from `40b0a3a` to `7a5a1c0`.
- [Commits](oxalica/rust-overlay@40b0a3a...7a5a1c0)

---
updated-dependencies:
- dependency-name: rust-overlay
  dependency-version: 7a5a1c0a5cb86a28224304309b68f050835fd1f6
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]