add support for AlwaysReplaceExistingDestPaths in llb copy #7308

Job	Run time
archutil-arm64	0s
prepare	7s
validate (lint-default)	11m 15s
validate (lint-labs)	11m 20s
validate (lint-nydus)	11m 35s
validate (lint-proto)	1m 5s
validate (lint-yaml)	28s
validate (validate-archutil)	35s
validate (validate-docs)	1m 1s
validate (validate-doctoc)	1m 6s
validate (validate-generated-files)	45s
validate (validate-shfmt)	19s
validate (validate-vendor)	34s
	40m 10s

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