update actions/checkout workflow to fix Node version warning

Author: Pathoschild

.github/workflows/build-smapi.yml

@@ -17,7 +17,7 @@ jobs:
             attestations: write
         steps:
             -   name: Checkout code
-                uses: actions/checkout@v4
+                uses: actions/checkout@v6
                 with:
                     fetch-depth: 0