sql: Add a couple of indexes.

Remove the Builds_index from the schema, as it would have been removed in
error by the upgrade-6 upgrade.  Add a specific index on the Builds status
field, as this helps with db-get-builds queries, and add an index on the
Outputs derivation field, as this helps with the db-get-outputs part of
db-get-builds.

* src/sql/upgrade-8.sql: New file.
* src/schema.sql: Update accordingly.