Fix Error 'Config validation error in build.os. Value build not found.'

[MaGering]

Building docs errors out:

Build # 23637095
[latest](https://readthedocs.org/dashboard/oemoflex/version/latest/edit/)
Build fehlgeschlagen
Error
Config validation error in build.os. Value build not found.

git clone --depth 1 https://github.com/rl-institut/oemoflex .
git fetch origin --force --prune --prune-tags --depth 50 refs/heads/dev:refs/remotes/origin/dev
git checkout --force origin/dev
git clean -d -f -f

With this PR the solution proposed according to issue readthedocs/readthedocs.org#11173
-> https://docs.readthedocs.io/en/stable/config-file/v2.html

[MaGering]

Can not test this since I can not create a version on readthedocs of this branch. Maybe the process is blocked because of the error on latest? I'm going to merg this PR and trigger the build for latest.