Actions: Fix "defaults" warning:

```
prepare-nonhindcast-cache
The 'defaults' channel might have been added implicitly. If this is intentional, add 'defaults' to the 'channels' list. Otherwise, consider setting 'conda-remove-defaults' to 'true'.
```

Author: simmsa

.github/workflows/main.yml

@@ -75,6 +75,7 @@ jobs:
           python-version: ${{ env.PYTHON_VER }}
           activate-environment: TESTconda
           use-only-tar-bz2: false
+          conda-remove-defaults: true
 
       - name: Create MHKiT Conda environment
         shell: bash -l {0}