Fix macOS compatibility: move Linux-only packages to conditional section in emu flake - #5

Fixes an issue where nix develop would fail on macOS with an error about parted not being available on arm64-apple-darwin.

Problem

The emu/flake.nix was unconditionally including parted and util-linux packages in the development shell, but these are Linux-only packages that don't exist on macOS platforms. This caused nix develop to fail with:

error: Package 'parted-3.6' in /nix/store/.../pkgs/by-name/pa/parted/package.nix:75 is not available on the requested hostPlatform:
  hostPlatform.config = "arm64-apple-darwin"
  package.meta.platforms = [
    "aarch64-linux"
    "armv5tel-linux"
    ...
  ]

Solution

Moved pkgs.parted and pkgs.util-linux from the unconditional packages list to the Linux-only conditional section, since:

1. The launch-pi.sh script already has platform-specific image mounting logic:

   • Linux: Uses losetup, mount, kpartx, and parted for loop device manipulation
   • macOS: Uses hdiutil for disk image operations

2. These tools are only needed for Linux-based image manipulation workflows

Changes

• Moved parted and util-linux to the pkgs.lib.optionals pkgs.stdenv.isLinux section
• Updated shell hook message to indicate parted is Linux-only
• Preserved all existing functionality and cross-platform compatibility

Now nix develop works on both Linux and macOS without requiring workarounds or environment variables.

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