Zarr: Update data variable attributes with `mode="a"`

What happened?

When appending a coordinate to an existing store, to_zarr will save the coordinate as a data variable.

What did you expect to happen?

to_zarr should record the appended item as a coordinate

Minimal Complete Verifiable Example

import numpy as np
import xarray as xr

da = xr.DataArray(np.ones(3), dims="x", name="a")
da.to_zarr("test")
da.coords["c"] = ("x", np.ones(3))
da.to_zarr("test", mode="a")
xr.open_zarr("test")

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

<xarray.Dataset> Size: 48B
Dimensions:  (x: 3)
Dimensions without coordinates: x
Data variables:
    a        (x) float64 24B dask.array<chunksize=(3,), meta=np.ndarray>
    c        (x) float64 24B dask.array<chunksize=(3,), meta=np.ndarray>

Anything else we need to know?

In the example I gave above this is happening because open_zarr expects the a zarr array to have an attribute called "coordinates" that is set to "c" but ZarrStore.set_variables doesn't save attributes for existing variables unless mode is set to "w".

Environment

Start with xarray/backends/zarr.py, especially ZarrStore.set_variables and the mode handling linked in the issue. Reproduce the provided to_zarr/open_zarr example, then add or update coverage for appending a coordinate and verify that reopening the store retains it as a coordinate.