metabase: fix restoring backups

In practice, the restoration was always failing with errors like:

  pg_restore: [archiver (db)] could not execute query: ERROR: schema "???" already exists

because the database was first initialized, but the usual restore
scenario is:
 - instance is populated with initial data
 - data is replaced by restoring the data

this also change the timeout to 5 seconds, 30 is too much