Set warnings as errors in docs build does not work correctly

[jenshnielsen]

For example https://github.com/QCoDeS/Qcodes/actions/runs/3575199370/jobs/6011431129

shows a build on 3.10 where sphinx warnings as errors should be true but there are still 4 warnings that do not cause the docs to build. This looks like an error in set-sphinx-opts since the docs build does trigger this line but the env variable is not updated