ci: update buildkit to v0.20.0-rc3 #5520

Job	Run time
prepare	1s
test / prepare	7m 55s
test / run (containerd, ./frontend/dockerfile, integration)	5m 17s
test / run (containerd, ./frontend/dockerfile, dockerfile)	7m 14s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	5m 7s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	7m 3s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	5m 15s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	7m 19s
test / run (containerd-1.7, ./frontend/dockerfile, integration)	5m 15s
test / run (containerd-1.7, ./frontend/dockerfile, dockerfile)	7m 23s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 36s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	7m 32s
test / run (oci, ./frontend/dockerfile, integration)	4m 52s
test / run (oci, ./frontend/dockerfile, dockerfile)	6m 12s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 38s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	6m 7s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 38s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	6m 12s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 18s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 22s
image (mainline)	5m 37s
image (labs)	5m 34s
scout	0s
release	0s
	2h 6m 27s

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