Build a list of Guix System images. For now, this list only contains the Hurd
barebones Guix System image.
* gnu/ci.scm (%guixsd-supported-systems): Remove "armhf-linux",
(%u-boot-systems): remove unused variable,
(%guix-system-images): new variable
(qemu-jobs): rename to "image-jobs" and build the Guix Systems listed in the
new "%guix-system-images" variable,
(system-test-jobs): adapt accordingly,
(hydra-jobs): ditto.
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