Update to actions/checkout@v6

Author: jan.nijtmans

.github/workflows/linux-build.yml

@@ -15,9 +15,9 @@ jobs:
           - "clang"
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Checkout tclconfig
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           repository: tcltk/tclconfig
           ref: main