workflow: bump actions/checkout from v2 to v4

Author: joonas-fi

.github/workflows/build.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - name: Build
       run: |