Flake: use upstream naersk

[veehaitch]

Upstream naersk now has support for

• aarch64-darwin: Add support for aarch64-darwin nix-community/naersk#182
• Stable Rust channel: Don't rely on unstable flags nix-community/naersk#191

That means, we no longer have to use our fork.

[veehaitch]

Resolved URL:  github:yaxitech/ragenix/switch-to-upstream-naersk
Locked URL:    github:yaxitech/ragenix/163aa564ab9590560a1512ea755f87e1ba9cd376
Description:   A rust drop-in replacement for agenix
Path:          /nix/store/hnxz7xa14apr8mcy4zhvglxjpjbyczzd-source
Revision:      163aa564ab9590560a1512ea755f87e1ba9cd376
Last modified: 2021-09-22 08:46:03
Inputs:
├───agenix: github:ryantm/agenix/daf1d773989ac5d949aeef03fce0fe27e583dbca
│   └───nixpkgs follows input 'nixpkgs'
├───flake-utils: github:numtide/flake-utils/7e5bf3925f6fbdfaf50a2a7ca0be2879c4261d19
├───naersk: github:nix-community/naersk/ee7edec50b49ab6d69b06d62f1de554efccb1ccd
│   └───nixpkgs follows input 'nixpkgs'
├───nixpkgs: github:nixos/nixpkgs/bc9b956714ed6eac5f8888322aac5bc41389defa
└───rust-overlay: github:oxalica/rust-overlay/d875f63082a3db25fa233d4ccd8f86b138c6cfeb
    ├───flake-utils follows input 'flake-utils'
    └───nixpkgs follows input 'nixpkgs'