Export step fails to create new branch

[pabloarosado]

I was trying to execute the export://github/co2_data/latest/owid_co2 step, by running:

CO2_BRANCH=update-gcb-data etlr github/co2_data --export

However, I encountered a few issues:

• OWIDBOT_ACCESS_TOKEN needed to be defined in .env. This was easy to fix, but I'm wondering if this was necessary. I suppose it should rather be using our GITHUB_TOKEN.
• I expected it would create a new branch, but instead it failed because the branch was not found. So I manually created the branch, rerun the export step, and then it worked.

Ideally, this step should:

• Not require the CO2_BRANCH variable. Instead, it would create a branch with the same name as the current ETL branch (and, if it already exists, it should fail, for safety).
• Create a branch and a PR.

[Marigold]

Fixed by #3605