Default processor.concurrency to 1 (#284) #59

Job	Run time
Build distribution binaries	5m 40s
Build Docker images (linux/386)	1m 38s
Build Docker images (linux/amd64)	1m 53s
Build Docker images (linux/amd64/v2)	1m 29s
Build Docker images (linux/amd64/v3)	1m 29s
Build Docker images (linux/arm/v6)	1m 25s
Build Docker images (linux/arm/v7)	1m 27s
Build Docker images (linux/arm64)	1m 32s
Build Docker images (linux/ppc64le)	1m 29s
Build Docker images (linux/s390x)	1m 28s
Publish Docker multi-arch manifest	9s
	19m 39s

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