limit amount of slot queue workers to 1 #3885

Job	Run time
matrix	6s
coverage	20m 29s
build / linux-amd64-pinned-all	1h 26m 5s
build / macos-amd64-pinned-all	1h 29m 2s
build / windows-amd64-pinned-unittest	35m 42s
build / windows-amd64-pinned-contract	1h 40m 17s
build / windows-amd64-pinned-integration	1h 40m 16s
build / windows-amd64-pinned-tools	23m 46s
build / status	1s
	7h 35m 44s

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