ci: fix cache key

Author: jcarpent

.github/workflows/macos-linux-conda.yml

@@ -29,7 +29,7 @@ jobs:
     - uses: actions/cache@v2
       with:
         path: ${{ env.CCACHE_DIR }}
-        key: ccache-${{ matrix.env.ROS_DISTRO }}-${{ matrix.env.ROS_REPO }}
+        key: ccache-${{ matrix.os }}
 
     - name: Install cmake and update conda
       shell: bash -l {0}