fix: remove all remaining uses of the term NixOS

Of particular importance is the change in the README to make it clear
that Nix is not so much a distribution as it is an universal package
distribution option on par with Flatpak and Snap.

README.md

@@ -28,7 +28,7 @@ a new machine or can't remember all the package names for your favorite apps?
 - Arch Linux (pacman + AUR via yay)
 - Fedora (dnf)
 - openSUSE (zypper)
-- NixOS (nix-env)
+- Nix (nix-env)
 - Flatpak
 - Snap
 
@@ -107,8 +107,8 @@ See [CONTRIBUTING.md](CONTRIBUTING.md) for contribution guidelines.
 ## 🎯 Roadmap
 
 ### Completed
-- [x] Multi-distro support (Ubuntu, Debian, Arch, Fedora, openSUSE, NixOS)
-- [x] Flatpak & Snap universal package support
+- [x] Multi-distro support (Ubuntu, Debian, Arch, Fedora, openSUSE)
+- [x] Nix, Flatpak & Snap universal package support
 - [x] 150+ applications across 15 categories
 - [x] Smart script generation with error handling
 - [x] AUR helper integration (yay) for Arch
@@ -139,4 +139,4 @@ Free software — you can redistribute and modify it under the terms of the GNU
 
 <p align="center">
 	<img src="https://raw.githubusercontent.com/catppuccin/catppuccin/main/assets/footers/gray0_ctp_on_line.svg?sanitize=true" />
-</p>
+</p>