Use ECMWF's downstream-ci in GitHub Actions

[iainrussell]

What maintenance does this project need?

To make earthkit-maps' 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 earthkit-data
• any ci actions that are special to this package should be in a separate yml file
• revise the cd action as per instructions here

Note: this cannot be done until the same is done for earthkit-data (ecmwf/earthkit-data#259).

Thank you!

Organisation

ECMWF