Hi Guix,

The attached patch fixes a bug where e.g.

  guix build -s i686-linux ffmpeg

builds a different derivation on i686-linux than on x86_64-linux.  This
doesn't just affect ffmpeg but a whole class of packages which use or
depend on a package that uses 'url-fetch/tarbomb' or 'url-fetch/zipbomb'
as the origin method of its source.  That's around 334 packages, among
them diffoscope, enlightenment, gnome, ungoogled-chromium, and wine.

The problem is fixed by explicitly passing the correct #:system and
#:guile-for-build to 'gexp->derivation' (as is done in other origin
methods such as 'git-fetch' or 'hg-fetch').

This shouldn't trigger any rebuils as it only affects the behaviour of
`guix build -s $system $package' if $system differs from the system type
of Guix itself.

Regards,

Diego