builder: return error if all nodes fail to boot #2902

Job	Run time
buildkit-edge	1m 59s
prepare-binaries	5s
prepare-test	2m 5s
binaries (darwin/amd64)	2m 48s
binaries (darwin/arm64)	2m 39s
binaries (linux/amd64)	2m 45s
binaries (linux/arm/v6)	2m 49s
binaries (linux/arm/v7)	2m 55s
binaries (linux/arm64)	2m 34s
binaries (linux/ppc64le)	2m 13s
binaries (linux/riscv64)	2m 18s
binaries (linux/s390x)	2m 9s
binaries (windows/amd64)	2m 12s
binaries (windows/arm64)	2m 17s
test (docker, ./tests)	5m 42s
test (docker\+containerd, ./tests)	5m 44s
test (docker-container, ./tests)	3m 41s
test (remote, ./tests)	5m 22s
test (./..., 1)	5m 33s
release	36s
bin-image	0s
	58m 26s

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