Bump nix from 0.19.1 to 0.20.0

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/commits)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Cargo.lock

@@ -7,5 +7,5 @@ authors = ["Jörg Thalheim <[email protected]>"]
 edition = "2018"
 
 [dependencies]
-nix = "0.19.*"
+nix = "0.20.*"
 libc = "*"