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>

readme.md

@@ -20,7 +20,7 @@ Join us on Matrix over at
 
 ### Compile-time
 
-- zig 0.16.x (you must use a **release version** of zig; check that `zig
+- zig 0.17.x (you must use a **release version** of zig; check that `zig
   version` does not have a `-dev*` suffix)
 
 - libc
@@ -51,16 +51,6 @@ Join us on Matrix over at
 # dnf install kernel-devel pam-devel libxcb-devel zig xorg-x11-xauth xorg-x11-server brightnessctl
 ```
 
-#### Nix
-
-You can use the flake to automatically install all dependencies inside a
-reproducible environment:
-
-```console
-$ git clone https://codeberg.org/fairyglade/ly.git && cd ly
-$ nix build
-```
-
 > [!WARNING]
 >
 > Distributions using SELinux such as Fedora and openSUSE Tumbleweed may
@@ -100,6 +90,17 @@ $ nix build
 # pkg install ca_root_nss libxcb git xorg xauth
 ```
 
+#### Nix
+
+Enter a development shell with all dependencies:
+
+```console
+$ git clone https://codeberg.org/fairyglade/ly.git && cd ly
+$ nix develop
+```
+
+Then follow [Manually building](#manually-building) from the `zig build` step.
+
 ## Availability
 
 [![Packaging status](https://repology.org/badge/vertical-allrepos/ly-display-manager.svg?exclude_unsupported=1)](https://repology.org/project/ly-display-manager/versions)