Incorrect time-slices after inserting into data cube (zarr) with packed variables

[AliceBalfanz]

Describe the bug
When an input slice is inserted into a data cube in zarr format which has defined a packing of variables, these input slices are incorrect in the data cube.

This problem is visualized in the viewer.cyanoalert.com where an input slice which has been inserted looks like this:

and if the same input slice has been added to a cube by appending it to the cube:

To Reproduce
Steps to reproduce the behavior:

1. Select a few input datasets
2. Create a data cube with defined packing, where xcube presorts the input slices
3. Create a data cube with defined packing and ensure, that xcube gets a wrong order of input slices and forcing it to insert.
4. Compare diff of both data cubes

Expected behavior
There should be no difference between the two cubes, xcube should be able to handle inserting of input slices with defined packing.