bump github.com/moby/buildkit from v0.10.3 to v0.12.3 #495

Job	Run time
prepare (mainline)	33s
prepare (buildkit)	10s
e2e (pip-youtube-dl, linux/arm/v7)	1m 32s
test (mainline)	4m 1s
e2e (webpack, linux/arm/v7)	2m 27s
validate (./hack/lint)	39s
test (buildkit)	39s
e2e (ejabberd, linux/arm64)	23m 43s
validate (./hack/validate-vendor)	21s
validate (./hack/install-and-test)	59s
build (mainline)	2m 33s
build (buildkit)	17s
	37m 54s

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