Use List.concat_map in MyARG.cartesian_concat_paths #14633

Job	Run time
regression (ubuntu-22.04, ocaml-variants.4.14.2+options,ocaml-option-flambda)	7m 36s
regression (macos-13, ocaml-variants.4.14.2+options,ocaml-option-flambda)	20m 32s
extraction	0s
	28m 8s

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