Update actions/cache action to v2.1.6

Author: renovate-bot

.github/workflows/ci.yaml

@@ -117,7 +117,7 @@ jobs:
           driver-opts: image=moby/buildkit:master
 
       - name: Cache Docker layers
-        uses: actions/cache@v2.1.5
+        uses: actions/cache@v2.1.6
         with:
           path: /tmp/.buildx-cache
           key: ${{ runner.os }}-buildx-${{ github.sha }}