add index build(package.id)
    
    This address:
    coprdb=# explain analyze SELECT package.id AS package_id, package.source_json AS source_json, build.pkg_version AS pkg_version, package.copr_id AS copr_id
    coprdb-#                 FROM package
    coprdb-#                 LEFT OUTER JOIN build ON build.package_id = package.id
    coprdb-#                 WHERE package.source_type = 5 AND
    coprdb-#                       package.source_json ~* '()' AND
    coprdb-#                       package.webhook_rebuild = true AND
    coprdb-#                       (build.id is NULL OR build.id = (SELECT MAX(build.id) FROM build WHERE build.package_id = package.id));
    
    before
     Planning time: 26.548 ms
     Execution time: 61695.949 ms
    after
     Planning time: 1.708 ms
     Execution time: 726.944 ms