Enable job discard, exporter stub, context cancel bug #557

Job	Run time
prepare	4s
test / prepare	1m 49s
binaries (darwin/amd64)	58s
binaries (darwin/arm64)	1m 2s
binaries (linux/amd64)	2m 0s
binaries (linux/arm/v7)	2m 12s
binaries (linux/arm64)	2m 5s
binaries (linux/s390x)	2m 7s
binaries (linux/ppc64le)	2m 22s
binaries (linux/riscv64)	2m 5s
binaries (windows/amd64)	1m 0s
binaries (windows/arm64)	1m 8s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	34m 38s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	34m 24s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	33m 33s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	34m 18s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	34m 41s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	34m 41s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	33m 43s
test / run (./..., 1, integration gateway)	34m 50s
test / run (./client, containerd, nydus, integration)	3m 59s
test / run (./client, oci, nydus, integration)	4m 1s
test / run (./..., nydus, 1, integration)	33m 29s
image	0s
release	0s
	5h 35m 9s

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