use conda for upstream dev uninstall again (#4706)

Author: mathause

ci/azure/install.yml

@@ -15,7 +15,7 @@ steps:
 # NumPy again: https://github.com/pydata/xarray/issues/4146 
 - bash: |
     source activate xarray-tests
-    mamba uninstall -y --force \
+    conda uninstall -y --force \
         numpy \
         scipy \
         pandas \