Force usage of conda-forge in CI

Author: djhoese

.github/workflows/ci.yaml

@@ -39,6 +39,9 @@ jobs:
           python-version: "${{ matrix.python-version }}"
           environment-file: continuous_integration/environment.yaml
           activate-environment: test-environment
+          channels: conda-forge
+          conda-remove-defaults: true
+          channel-priority: strict
 
       - name: Install unstable dependencies
         if: matrix.experimental == true