ci: update buildkit to v0.20.0-rc3 #6170

Job	Run time
build (windows/amd64)	3m 55s
build (freebsd/amd64)	2m 56s
sandbox-build (linux/amd64)	6m 55s
sandbox-build (linux/arm64)	6m 42s
test-freebsd-amd64	2m 46s
test-windows-amd64 (windows-2022, containerd, ./client)	7m 45s
test-windows-amd64 (windows-2022, containerd, ./cmd/buildctl)	1m 57s
test-windows-amd64 (windows-2022, containerd, ./worker/containerd)	2m 59s
test-windows-amd64 (windows-2022, containerd, ./solver)	2m 43s
test-windows-amd64 (windows-2022, containerd, ./frontend)	1m 39s
test-windows-amd64 (windows-2022, containerd, ./frontend/dockerfile)	39m 42s
test-windows-amd64 (windows-2025, containerd, ./client)	5m 48s
test-windows-amd64 (windows-2025, containerd, ./cmd/buildctl)	2m 16s
test-windows-amd64 (windows-2025, containerd, ./worker/containerd)	1m 52s
test-windows-amd64 (windows-2025, containerd, ./solver)	2m 14s
test-windows-amd64 (windows-2025, containerd, ./frontend)	2m 24s
test-windows-amd64 (windows-2025, containerd, ./frontend/dockerfile)	39m 43s
test-windows-amd64 (windows-2022, containerd, ./..., 1)	4m 38s
test-windows-amd64 (windows-2025, containerd, ./..., 1)	4m 7s
	2h 23m 1s

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