Use zig 0.17 in the Nix devShell and update the readme (#1067)

## What are the changes about?

Update the nixpkgs input in flake.lock, since zig_0_17 is not available in the previous revision, use it in the devShell, bump the zig version in the readme to 0.17.x, and replace the nix build instructions (the flake has no packages output) with nix develop.

## What existing issue(s) does this resolve?

N/A

## Pre-requisites

- [x] I have tested & confirmed the changes work locally
- [x] I have read and fully adhere to the rules set in the contributing guidelines found in `CONTRIBUTING.md`

Reviewed-on: https://codeberg.org/fairyglade/ly/pulls/1067
Reviewed-by: AnErrupTion <anerruption+codeberg@disroot.org>

flake.nix

@@ -21,7 +21,7 @@
         {
           default = pkgs.mkShell {
             packages = with pkgs; [
-              zig
+              zig_0_17
               pam
               libxcb
             ];