%build-inputs was not defined anymore when cross-compiling.
This causes some build failures on core-updates, see e.g.
<https://ci.guix.gnu.org/build/653883/log/raw>;.  According
to civodul on IRC, this removal of %build-inputs was
accidental, so re-introduce it.

This allows "guix build grep --target=aarch64-linux-gnu" to
succeed on a x86_64-linux machine.  The resulting binary
is executable under qemu-aarch64.