ci(Makefile): remove build-related env variables #1296

Job	Run time
console / helm-integration-test-console	16m 7s
backend (artifact-backend) / helm-integration-test	12m 0s
backend (mgmt-backend) / helm-integration-test	11m 57s
backend (model-backend) / helm-integration-test	15m 29s
backend (pipeline-backend) / helm-integration-test	15m 25s
	1h 10m 58s

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