iF you have a target that builds your code and you decide you want to run it with GOOS=windows ... Mage will try to compile the magefile with GOOS=windows, and if you're not on windows, it'll break.

i.e.

GOOS=windows mage build

will compile the magefile itself with GOOS=windows ... and then try to run it and fail if you're not on windows.

We need to isolate mage's own compilation from the standard environment variables, with the possibility of something like MAGEFILE_GOOS for when you want to produce a magefile binary of a specific os/arch.