Document why binary build uses production #5260

Job	Run time
build-and-check	2m 45s
test-web	38s
test-common	39s
test-integration-web	42s
test-sdk	55s
test-cli	1m 12s
test-integration-packages/agent-runtime	39s
test-packages/agent-runtime	55s
test-integration-common	47s
test-integration-packages/internal	45s
test-packages/billing	46s
test-.agents	39s
test-integration-sdk	42s
test-integration-cli	48s
test-packages/internal	47s
test-integration-packages/billing	40s
test-integration-.agents	42s
	15m 1s

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