limit amount of slot queue workers to 1 #3886

Job	Run time
matrix	17s
coverage	20m 13s
build / linux-amd64-pinned-all	1h 9m 14s
build / macos-amd64-pinned-all	1h 9m 20s
build / windows-amd64-pinned-unittest	35m 7s
build / windows-amd64-pinned-contract	1h 9m 18s
build / windows-amd64-pinned-integration	1h 9m 16s
build / windows-amd64-pinned-tools	22m 37s
build / status	1s
	5h 55m 23s

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