Fix race condition for multiple very fast downloads #1312

Job	Run time
build_NetCore (Debug_NetCore)	1m 21s
build (6.8, Debug)	2m 1s
build_NetCore (Release_NetCore)	1m 14s
build (6.8, Release)	3m 1s
build (6.10, Debug)	1m 42s
build (6.10, Release)	1m 49s
build (6.12, Debug)	1m 42s
build (6.12, Release)	2m 3s
build (latest, Debug)	1m 23s
build (latest, Release)	2m 5s
	18m 21s

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