hydra.sql: add an index for slow queries in production

These queries used to use (jobset, project) based indexes,
and the addition of jobset_id makes most of those indexes
unusable now.

src/sql/upgrade-65.sql

@@ -0,0 +1,2 @@
+-- Add an index like IndexBuildsOnJobFinishedId using jobset_id
+create index IndexBuildsOnJobsetIdFinishedId on Builds(id DESC, finished, job, jobset_id);