fix: support adding dependencies for project's unsupported platforms

[orhun]

fixes #650

reproducible case (on Linux):

channels = ["conda-forge"]
platforms = [
  # "linux-64",
  "osx-arm64",
]
name = "bug"

[dependencies]
python = "=3.11.*"

Run:

$ pixi add --no-install ruff

 WARN pixi::environment: Adding dependency for unsupported platform (linux-64).
✔ Added ruff

ruff = ">=0.1.13,<0.2" added to pixi.toml 🎉

[ruben-arts]

I think it would be nice to automate this when the current platform is not specified.
e.g.

[project]
name = "not_my_platform_project"
platforms = ["osx-arm64"]
channels = ["conda-forge"]

Would trigger the "--no-install" logic with that warning on the other platforms.

[orhun]

Do you mean running pixi add xyz on an unsupported platform would only add the dependency but not install it?

In that case we remove the "The project needs to be configured..." error, right?

[orhun]

I YOLO'd it in 1cce968

Let me know if the behavior is correct + if you like the mutable reference. (we can return bool from sanity_check_project instead)