database: Reset timestamps to one second after the Epoch.

database: Reset timestamps to one second after the Epoch.

Previously, store items registered in the database by this code (for
instance, store items retrieved by 'guix offload' and passed to
'restore-file-set') would have an mtime of 0 instead of 1.

This would cause problems for things like .go files: Guile would
consider them to be older than the corresponding .scm file, and
consequently it would ignore them and possibly use another (incorrect)
.go file.

Reported by Ricardo Wurmus.

* guix/store/database.scm (reset-timestamps): Pass 1, not 0, to
'utime'.
* tests/store-database.scm ("register-path"): Check the mtime of FILE
and REF.