gexp: #:references-graphs refers to non-native derivations.

gexp: #:references-graphs refers to non-native derivations.

Fixes a regression introduced in
c6d6aee6659acb293eb33f498fdac3b47a19a48, where #:reference-graphs would
end up referring to native inputs.

This would notably break the compilation of systems using a childhurd,
because they would attempt to build the 'hurd' package natively.

* guix/gexp.scm (lower-reference-graphs)[tuple->gexp-input]: Honor TARGET.
* tests/gexp.scm ("gexp->derivation #:references-graphs cross-compilation"):
New test.