use python-only environment file for Circle build documentation (#3981)

Author: valeriupredoi

.circleci/config.yml

@@ -247,7 +247,7 @@ jobs:
             mkdir /logs
             . /opt/conda/etc/profile.d/conda.sh
             # Install
-            mamba env create
+            mamba env create -n esmvaltool -f environment_python.yml
             conda activate esmvaltool
             pip install .[doc]
             # Log versions