Need a distinct cache key for ccache as well.

.github/workflows/ci.yml

@@ -123,7 +123,7 @@ jobs:
         uses: actions/cache@v2
         with:
           path: ~/.ccache/
-          key: ${{ runner.os }}-ccache
+          key: ${{ runner.os }}-ccache-${{ matrix.python-version }}
 
       - name: Get pip cache dir
         id: pip-cache