Update dependencies to use nix >= 0.23 due to vulnerability

[eldruin]

See: https://rustsec.org/advisories/RUSTSEC-2021-0119.html

• Release 0.5.1 rust-i2cdev#69
• Release 0.5.1 rust-spidev#37
• Release 0.5.1 gpio-cdev#67
• Release 0.6.1 rust-sysfs-gpio#72
• Release 0.2.0 milkey-mouse/openpty#7 (dev-dependency, optional)
• TODO: linux-embedded-hal

[ryankurte]

all lgtm, though i can't see that the particular security issue has any impact in our crates.

[eldruin]

Thanks! Indeed it does not seem so. However, anything that depends on it starts getting flagged by cargo-audit.
Since it is the first time this comes up, I figured we might as well update.

[posborne]

Thanks for picking this one up and running with it @eldruin