etl run --export should default to a matching branch for other repos

[larsyencken]

Problem

In our ETL staging work, we are moving to using matching branches (my-data-change) on both the ETL and owid-content.

Currently our etl run --export does a dry-run push to master on owid-content and other repositories, such as co2-data. You use an environment variable to pick which branch to do a real push to. But actually we should default to the matching branch name in the external repo.

This will make them work more harmoniously with the new staging flow.

[larsyencken]

Recall:

• We have external namespace, to remind us that someone else is using this
• We have export, which is meant to be for things we push elsewhere
• We want to turn the OWID external steps into export steps, meaning that at the ETL you have control over publishing that data

[Marigold]

This has been implemented for etlr github/co2_data --export step in this PR #3605. Other steps could use the same pattern.