bug#40381: Guix shouldn't request substitutes for profile derivations

[pkill9]

Hi Ludovic

> Can you show more precisely what you mean by pasting a command and its
> output?
> 
> With the recent changes in the implementation of grafts, what happens
> is usually this:
> 
>   $ guix build foo
>   updating the list of substitutes…
>   The following things will be built/downloaded:
>   …
> 
>   updating the list of substitutes…
>   The following things will be built/downloaded:
>   …
> 
> The second stage here typically includes profile.drv as well as
> grafts. All this is expected behavior.

I think that since profile.drv is always produced locally on the
machine, then it's unnecessary to update the list of substitutes
when it's on that second stage.