cosmicds: Fix logs not being generated

Turns out us unconditionally setting `jupyterhub-singleuser`
as `cmd` prevents repo2docker's automatic log generation from
working!

Ref 2i2c-org#3109

config/clusters/2i2c-aws-us/cosmicds.values.yaml

@@ -37,6 +37,9 @@ jupyterhub:
           name: Cosmic DS, Harvard
           url: https://www.cosmicds.cfa.harvard.edu/
   singleuser:
+    # Unset cmd so repo2docker-entrypoint gets used, and logs actually work
+    # Ref https://github.com/2i2c-org/infrastructure/issues/3109
+    cmd: null
     # No persistent storage should be kept to reduce any potential data
     # retention & privacy issues.
     # Ref https://github.com/2i2c-org/infrastructure/issues/2128#issuecomment-1635107926