Use ECMWF's downstream-ci in GitHub Actions

[iainrussell]

What maintenance does this project need?

To make earthkit-data's ci work in the same way as other ECMWF packages, the following should be done:

• add to downstream-ci as per instructions here
• dependencies in the tree for now should be eccodes-python, cfgrib and pdbufr; very likely pyfdb and pyodc should be there too; for now, earthkit-meteo can be taken from PyPi
• any ci actions that are special to this package (e.g. the no-eccodes environment) should be in a separate yml file
• revise the cd action as per instructions here

Thank you!

Organisation

ECMWF

[sandorkertesz]

Fixed by #262