fix: --import env_file potentially breaks channel priority

[fecet]

The current implementation updates the extra_channels read from dependencies using the channels read from envfile, which introduces an unnecessary assumption that the entries in dependencies need to be arranged in the expected order of priority.

This may not happen in many cases. Generally speaking, channels in the env_file can be considered sorted, while dependencies are unordered. Additionally, if new channels appear in extra_channels, placing them at the end of the priority list should be a safer default behavior.

Consider the following example from https://pixi.sh/dev/advanced/channel_priority/#use-case-pytorch-and-nvidia-with-conda-forge, which generates an incorrect pixi.toml:

channels:
  - nvidia/label/cuda-11.8.0
  - nvidia
  - conda-forge
  - pytorch
  - nodefaults
dependencies:
  - pytorch::pytorch 2.0.1.*
  - pytorch::torchvision 0.15.2.*
  - pytorch::pytorch-cuda 11.8.*
  - nvidia/label/cuda-11.8.0::cuda *
  - python 3.10.*

pixi init --import will result in

[project]
authors = ["fecet <[email protected]>"]
channels = ["pytorch", "nvidia/label/cuda-11.8.0", "nvidia", "conda-forge", "nodefaults"]
description = "Add a short description here"
name = "import-bug"
platforms = ["linux-64"]
version = "0.1.0"

[tasks]

[dependencies]
pytorch = { version = "2.0.1.*", channel = "pytorch" }
torchvision = { version = "0.15.2.*", channel = "pytorch" }
pytorch-cuda = { version = "11.8.*", channel = "pytorch" }
cuda = { version = "*", channel = "nvidia/label/cuda-11.8.0" }
python = "3.10.*"

[ruben-arts]

Thank you this would indeed be a better assumption!