Prevent native-inputs references ending up in the final binary

[Danny Milosavljevic]

Hi Leo,

> Although native-inputs are typically things that are only required while
> building [0], there's nothing that prevents a built package from keeping
> references to native-inputs.

We should change that in core-updates-next, if possible.
 
I think that native-inputs shouldn't end up in the final binary as a
reference, especially when cross-compiling
(but we don't do cross-compilation much in Guix - usually, we let
qemu-arm emulate the ARM CPU on x86_64 and just call the target tool :) ).

If there are indeed parts of the same package, one a native part and one a
runtime dependency part, I actually write the same package reference twice,
once in the inputs, once in the native-inputs, in my custom package
definitions.

In a "previous life" I did a lot of Linux cellphone development and,
there, it was kinda important that a x86_64 toolchain doesn't end
up being referenced in an ARM binary, so the habit stuck - and I
think it's important to distinguish the mold used to form a product
from an integral part of that product.