cpplint: use cpplint version 1.6.1 for now

when upgrading to cpplint 2.0.0, C-style casts are flagged. since many
of them are in libraries, we will use version 1.6.1 until we figure out
whether we shall ignore C-style casts or if we will replace them.

.github/workflows/cpplint.yml

@@ -19,7 +19,9 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install cpplint
+          pip install cpplint==1.6.1
       - name: Linting
         run: |
-          cpplint --repository=. --recursive --filter=-build/c++11,-runtime/references,-readability/braces,-whitespace,-legal,-build/include ./src ./include ./lib/Hoymiles ./lib/MqttSubscribeParser ./lib/TimeoutHelper ./lib/ResetReason
+          cpplint --repository=. --recursive \
+            --filter=-build/c++11,-runtime/references,-readability/braces,-whitespace,-legal,-build/include \
+            ./src ./include ./lib/Hoymiles ./lib/MqttSubscribeParser ./lib/TimeoutHelper ./lib/ResetReason