Ignore config.system.indexBuilds collection

See https://forums.percona.com/t/pbm-backup-error-in-a-replicaset-non-sharded-scenario/11709/4 and https://jira.mongodb.org/browse/TOOLS-2537.

Although mongo-tools are updated to include the following patch, the oplog still would try to save and replay this collection. 

Should be fixed