limit amount of slot queue workers to 1 #3904

Job	Run time
matrix	20s
coverage	19m 1s
build / linux-amd64-pinned-all	1h 17m 26s
build / macos-amd64-pinned-all	1h 17m 27s
build / windows-amd64-pinned-unittest	35m 58s
build / windows-amd64-pinned-contract	1h 17m 24s
build / windows-amd64-pinned-integration	1h 17m 25s
build / windows-amd64-pinned-tools	23m 31s
build / status	1s
	6h 28m 33s

Provide feedback