[MLIR][Linalg] Respect DPS in `lower_unpack`

`tensor.unpack` implements the DPS (Destination Passing Style) interface
and expects the result to be "stored" in the `outs` operand, but this is
not the case with the current decomposition as the final operation is a
`tensor.extract_slice` that does not implement DPS. Add a `linalg.copy`
to fix the problem.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D158393