ci: do not use matrix for code_style_check

So the job can be used as 'required' for PRs

.github/workflows/code_style_check.yaml

@@ -4,16 +4,8 @@ on: [pull_request]
 
 jobs:
   code_style_check:
-    strategy:
-      matrix:
-        otp:
-          - 24.2.1-1
-        elixir:
-          - 1.13.3
-        os:
-          - ubuntu20.04
     runs-on: ubuntu-20.04
-    container: "ghcr.io/emqx/emqx-builder/5.0-10:${{ matrix.elixir}}-${{ matrix.otp }}-${{ matrix.os }}"
+    container: "ghcr.io/emqx/emqx-builder/5.0-10:1.13.3-24.2.1-1-ubuntu20.04"
     steps:
       - uses: actions/checkout@v2
         with: