[zbus-1] Update nix to 0.20
Get fix for RUSTSEC-2021-0119
Closes #212 (closed)
See https://rustsec.org/advisories/RUSTSEC-2021-0119 and https://github.com/nix-rust/nix/issues/1541
Once this is done, a new version of zbus
1.9 should be published
Edited by Nixon Enraght-Moony