deduplication: pass store directory to replace-with-link.

deduplication: pass store directory to replace-with-link.

This causes with-writable-file to take into consideration the actual store
being used, as passed to 'deduplicate', rather than
whatever (%store-directory) may return.

* guix/store/deduplication.scm (replace-with-link): new keyword argument
  'store'.  Pass to with-writable-file.
  (with-writable-file, call-with-writable-file): new store argument.
  (deduplicate): pass store to replace-with-link.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>