Image files can be very large (multiple gigabytes), and the process of
generating them is primarily I/O bound. As disk access is typically faster
than network access, it makes sense build them locally.
* gnu/system/image.scm (system-disk-image): Pass the #:local-build? #t
parameter to computed-file calls dealing with generating image files.
(system-iso9660-image): Likewise.
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