unzip fails to build when cross-compiling (at least from x86_64 
linux), complaining that '%output' is unbound.  It gives identical 
errors when compiling for aarch64, riscv64, and arm. 
Interestingly, it gives the same errors when explicitly building 
for x86_64 on an x86_64 machine, even though I would expect doing 
so to compile as normal.  When not cross-compiling, unzip 
successfully builds as normal on both x86_64 and aarch64.

On my x86_64 machines, 'guix show unzip' does only have 
x86_64-linux and i686-linux in the 'systems' list, but that's also 
true of htop, which does cross-compile.  (Also, why does it do 
that?  The same command on my aarch64 machine shows many more 
system types.)

I'm working on testing if cross-compiling from aarch64 does the 
same thing, but building a cross-compilation toolchain on pinebook 
pro is very slow.

Steps to reproduce: run 'guix build unzip --target=$TARGET'

Here's the build log from 'guix build unzip 
--target=aarch64-linux-gnu':