ci(system-tests): avoid rebuilding ddtrace from source #208

Job	Run time
build	1m 9s
system-tests / Get parameters / Get parameters	2m 10s
system-tests / Build end-to-end (apigw-http)	1m 13s
system-tests / Build end-to-end (alb-multi)	1m 9s
system-tests / Build end-to-end (apigw-rest)	1m 7s
system-tests / Build end-to-end (alb)	1m 13s
system-tests / parametric	0s
system-tests / Build end-to-end (function-url)	1m 11s
system-tests / End-to-end #1 / function-url 1	1m 25s
system-tests / End-to-end #1 / alb 1	1m 19s
system-tests / End-to-end #1 / alb-multi 1	1m 23s
system-tests / End-to-end #1 / apigw-http 1	1m 30s
system-tests / End-to-end #1 / apigw-rest 1	1m 34s
system-tests / Report failures	-1s
	16m 22s

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