ci(workflows): Update actions/checkout to version 6

Author: seagle0128

.github/workflows/ci.yml

@@ -31,7 +31,7 @@ jobs:
       - name: setenv
         run: echo "ACTIONS_ALLOW_UNSECURE_COMMANDS=true" >> $GITHUB_ENV
 
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - uses: jcs090218/setup-emacs@master
         with: