CI: Update checkout action

Author: sosthene-nitrokey

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
           - x86_64-unknown-linux-gnu
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Install build dependencies
         shell: bash