Format keyword "nc" for save_dataset issues an error

[mraspaud]

Describe the bug
When saving a dataarray as netcdf with the cf writer, providing the format="nc" keyword argument raises a ValueError: unrecognized format requested

This was reported first in pytroll/trollflow2#189

To Reproduce

    from satpy import Scene
    from glob import glob
    import os
    from satpy.utils import debug_on
    debug_on()
    datadir = "/home/a001673/data/satellite/Meteosat-11/seviri/lvl1.5/2018/02/28/HRIT"

    filenames = glob(os.path.join(datadir, "*2018*"))

    scn = Scene(filenames=filenames,
                reader='seviri_l1b_hrit',
                )


    composites = ["IR_087"]
    scn.load(composites)
    rscn = scn.resample("eurol")
    rscn.save_dataset(composites[0], "IR_087.nc", format="nc", writer="cf")

Expected behavior
I expect the file to we written without error. Removing format from the kwargs makes it work again.

Actual results
A ValueError is raised.

Environment Info:

• OS: RHEL 9.3
• Satpy Version: main from 2023-12-12