Use clang-format 6.0 to check for code style (#15403, #15404)

This way, the CI can run on the faster and more common Ubuntu machine.

.github/workflows/codestyle.yml

@@ -5,13 +5,14 @@ on: [push, pull_request]
 jobs:
 
   codestyle:
-    runs-on: macos-latest
+    runs-on: ubuntu-18.04
     steps:
     - uses: actions/checkout@v2
-    - run: brew install clang-format
+    - run: sudo apt install -y clang-format-6.0
       name: Install clang-format
-    - run: clang-format --version
+    - run: clang-format-6.0 --version
     - run: bash ./tools/format-code.sh
       name: Run code formatter
+    - run: git diff
     - run: git diff --quiet HEAD
       name: Check if the styling guide is followed

tools/format-code.sh

@@ -1,4 +1,4 @@
 #!/usr/bin/env bash
 
 cwd=$(pwd)
-clang-format -i --style=WebKit $cwd/src/*.h $cwd/src/*.cpp
+clang-format-6.0 -i --style=WebKit $cwd/src/*.h $cwd/src/*.cpp