fix: check for correct platform in task env creation

[ruben-arts]

pixi run -e mlx test would have tried to do something without an error. This pr changes that to this error:

➜ pixi run -e mlx test
  × The current platform 'linux-64' is not supported by the `mlx` environment. Supported platforms: osx-arm64

[project]
platforms = ["win-64", "linux-64", "osx-64", "osx-arm64"]

[tasks]
train = "python train.py"
test = "python test.py"
start = {depends_on = ["train", "test"]}


[feature.mlx]
platforms = ["osx-arm64"]
system-requirements = {macos = "13.3"}

[feature.mlx.tasks]
train = "python train.py --mlx"
test = "python test.py --mlx"

[feature.mlx.dependencies]
mlx = "*"


[environments]
mlx = ["mlx"]
default = ["cpu"]