Allow esmvaltool to download when missing

[sverhoeven]

We mount a dcache volume which contain climate data like era5. Esmvaltool can download climate data when missing. This needs creds described at https://docs.esmvaltool.org/projects/ESMValCore/en/latest/quickstart/configure.html#esgf-configuration

We could add central credentials or each user needs to set it up themselves.
We should check if centralized creds are allowed.

[BSchilperoort]

esmvaltool cannot download era5 though, we have to do that ourselves. Sadly the climate data store is currently unusable.

No creditials are needed for some models, such as EC-Earth3. The default option is to never download, we should tell users to look at the configuration and set search_esgf: when_missing in the esmvaltool configuration. Or we change that setting in infra.