limit amount of slot queue workers to 1 #3887

Job	Run time
matrix	16s
coverage	21m 10s
build / linux-amd64-pinned-all	1h 22m 24s
build / macos-amd64-pinned-all	1h 40m 19s
build / windows-amd64-pinned-unittest	22m 37s
build / windows-amd64-pinned-contract	1h 38m 20s
build / windows-amd64-pinned-integration	1h 38m 18s
build / windows-amd64-pinned-tools	17m 43s
build / status	1s
	7h 21m 8s

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