There is currently a window of time between when the build outputs are exposed
and when their metadata is canonicalized.
* nix/libstore/build.cc (DerivationGoal::registerOutputs): wait until after
metadata canonicalization to move successful build outputs to the store.
Change-Id: Ia995136f3f965eaf7b0e1d92af964b816f3fb276
Signed-off-by: Ludovic Courtès <ludo@gnu.org>