fs: fix incorrect dereferencing of src arg in cpSync #1415

Job	Run time
Build slim tarball	1m 18s
x86_64-darwin: with shared libraries	44m 7s
x86_64-linux: with shared libraries	29m 39s
aarch64-darwin: with shared libraries	28m 4s
aarch64-linux: with shared libraries	26m 37s
	2h 9m 45s

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