* gnu/system/vm.scm (expression->derivation-in-linux-vm): Remove #:target.
[builder]: Use #+. Don't pass #:target-arm32? and #:target-aarch64? to
'load-in-linux-vm'.
Pass #:target #f to 'gexp->derivation'.
(qemu-image): Adjust accordingly.
* gnu/build/vm.scm (load-in-linux-vm): Remove #:target-aarch64?
and #:target-arm32?. Define them as local variables.