system: image: Remove "image-root" derivation when building ISO.

The "image-root" derivation output is used as a temporary directory that is
passed to GNU Xorriso later on. By merging the creation of this directory and
the production of an ISO image, we can get rid of the derivation.

* gnu/system/image.scm (system-iso9660-image): Merge "image-root" and
"iso9660-image" derivations so that we spare an extra derivation.