Bump nix from 0.19.1 to 0.20.0 (#3587)

Bumps [nix](https://github.com/nix-rust/nix) from 0.19.1 to 0.20.0.
- [Release notes](https://github.com/nix-rust/nix/releases)
- [Changelog](https://github.com/nix-rust/nix/blob/master/CHANGELOG.md)
- [Commits](https://github.com/nix-rust/nix/compare/v0.19.1...v0.20.0)

---
updated-dependencies:
- dependency-name: nix
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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