add support for AlwaysReplaceExistingDestPaths in llb copy #7309

Job	Run time
prepare	7s
archutil-arm64	0s
validate (lint-default)	12m 34s
validate (lint-labs)	12m 51s
validate (lint-nydus)	12m 33s
validate (lint-proto)	28s
validate (lint-yaml)	33s
validate (validate-archutil)	31s
validate (validate-docs)	51s
validate (validate-doctoc)	36s
validate (validate-generated-files)	42s
validate (validate-shfmt)	15s
validate (validate-vendor)	37s
	42m 38s

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