Upgrade actions/checkout to v6

Author: theypsilon

.github/workflows/ci_build.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v6
 
     - name: Build
       run: set -o pipefail && curl --fail --location https://raw.githubusercontent.com/MiSTer-unstable-nightlies/Build-Automation_MiSTer/main/build.sh | bash -