Fix caching of conda env in CI (#218)

Author: blink1073

.github/workflows/tests.yml

@@ -40,7 +40,7 @@ jobs:
           path: ~/conda_pkgs_dir
           key:
             ${{ runner.os }}-conda-${{ env.CACHE_NUMBER }}-${{
-            hashFiles('etc/example-environment.yml') }}
+            hashFiles('docs/environment.yml') }}
       - name: Setup conda ${{ matrix.PYTHON_VERSION }}
         uses: conda-incubator/setup-miniconda@v2
         with: