Disable default features on nix crate

[hdoordt]

This PR disables the default features on the nix crate. Enabling default features makes nix, and therefore anything using redis-module, require at least glibc >= 2.27 as it'll contain a reference to e.g. memfd_create. For instance, Amazon Linux 2 ships with glibc 2.26 and does not work with default features enabled.

No features are actually used by redis-module.

[DvirDukhan]

@ephraimfeldblum LMK if it works for JSON

[ephraimfeldblum]

@DvirDukhan sure, no reason why JSON would object.

nix is only being used by redismodule to write to a raw fd in the event that the RedisAllocator is unavailable. write has be standardized since POSIX.1, I doubt you'll find an arch that doesn't support it.

[hdoordt]

@ephraimfeldblum @DvirDukhan Are we ready to merge this one?