Re-add jobset pattern index as it's actually still being used (#4428)

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Author: masipauskas

internal/lookout/schema/migrations/020_readd_jobset_index.sql

@@ -0,0 +1 @@
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_job_jobset_pattern ON job (jobset varchar_pattern_ops) WITH (fillfactor = 80);