limit amount of slot queue workers to 1 #3906

Job	Run time
matrix	21s
coverage	18m 53s
build / linux-amd64-pinned-all	1h 37m 16s
build / macos-amd64-pinned-all	1h 52m 0s
build / windows-amd64-pinned-unittest	34m 45s
build / windows-amd64-pinned-contract	19m 38s
build / windows-amd64-pinned-integration	1h 40m 5s
build / windows-amd64-pinned-tools	22m 9s
build / status	1s
	6h 45m 8s

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