Add missing defaults channel by last (see #69)

gitlab/before_build.sh

@@ -32,6 +32,7 @@ set_conda_channels ${VISIBILITY} ${CI_COMMIT_TAG}
 for k in "${CONDA_CHANNELS[@]}"; do
   echo "  - ${DOCSERVER}/${k}" >> ${CONDARC}
 done
+echo "  - defaults" >> ${CONDARC}
 
 # displays contents of our configuration
 echo "Contents of \`${CONDARC}':"