daemon: Delegate deduplication to 'guix substitute'.

daemon: Delegate deduplication to 'guix substitute'.

This removes the main source of latency between subsequent downloads.

* nix/libstore/build.cc (SubstitutionGoal::tryToRun): Add a
"deduplicate" key to ENV.
(SubstitutionGoal::finished): Remove call to 'optimisePath'.
* guix/scripts/substitute.scm (process-substitution)[destination-in-store?]
[dump-file/deduplicate*]: New variables.
Pass #:dump-file to 'restore-file'.
* guix/scripts/substitute.scm (guix-substitute)[deduplicate?]: New
variable.
Pass #:deduplicate? to 'process-substitution'.
* guix/serialization.scm (dump-file): Export and augment 'dump-file'.