Move the cow-store procedure from the service declaration in (gnu system
install) to (gnu build install), so that it can be called from within a
different context than Shepherd.
* gnu/build/install.scm (mount-cow-store, unmount-cow-store): New procedures.
* gnu/system/install.scm (make-cow-store): Remove it,
(cow-store-service-type): adapt it accordingly.
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