Use the named var for build dir #2641

Job	Run time
Basic Checks	3m 18s
Unit Tests	6m 7s
Integration Tests (raft)	11m 51s
Integration Tests (pvtdata)	11m 36s
Integration Tests (pvtdatapurge)	5m 32s
Integration Tests (ledger)	12m 31s
Integration Tests (lifecycle)	5m 0s
Integration Tests (e2e smartbft)	20m 40s
Integration Tests (discovery gossip devmode pluggable)	6m 15s
Integration Tests (gateway idemix pkcs11 configtx configtxlator)	16m 34s
Integration Tests (sbe nwo msp)	8m 10s
	1h 47m 34s

Provide feedback