Add environment variable to change Jetstream version #236

Job	Run time
prelim	7s
tpu_image / Build and upload image (v4-8)	1m 44s
gpu_image / Build and upload image (a100-40gb-4)	40s
gpu_unit_tests / run	7m 26s
gpu_integration_tests / run	7m 58s
tpu_integration_tests / run	6m 28s
tpu_unit_tests / run	13m 52s
Clean up	9s
	38m 24s

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