Avoid to update conda base environment with the defaults channel.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>

.gitlab-ci.yml

@@ -53,7 +53,7 @@ test_py_tools_ds_install:
     - source activate py_tools_ds_testinstall
 
     # update conda and python
-    - conda update -n base -c defaults conda
+    - conda update -n base -c conda-forge conda
     # - conda update -c conda-forge python
 
     # avoid package incompatibilities due to usage of wrong channels