* gnu/system/shadow.scm (<user-account>)[inputs]: New field.
(passwd-file): Bind the 'shell' field of each account.
* gnu/system/vm.scm (%demo-operating-system): Remove 'shell' field.
* gnu/system/dmd.scm (guix-build-accounts): Store a monadic value in
'shell'. Add 'inputs' field.
* gnu/system.scm (operating-system-derivation): Remove 'shell' field for
'root' account. Add all the 'user-account-inputs' to EXTRAS.
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