Merge pull request #4346 from tonistiigi/fix-cache-export-concurrency #1684

Job	Run time
prepare	3s
test / prepare	2m 44s
test / run (containerd, ./frontend/dockerfile, integration)	7m 8s
test / run (containerd, ./frontend/dockerfile, dockerfile)	14m 2s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	8m 43s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	8m 57s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	7m 18s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	9m 7s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	7m 17s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	16m 9s
test / run (oci, ./frontend/dockerfile, integration)	6m 14s
test / run (oci, ./frontend/dockerfile, dockerfile)	7m 50s
test / run (oci-rootless, ./frontend/dockerfile, integration)	5m 48s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	9m 10s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	6m 18s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	8m 15s
image (mainline)	11m 1s
image (labs)	9m 40s
release	0s
	2h 25m 44s

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