GitHub-CI: Bump actions/checkout from 5 to 6.

* .github/workflows/make-cross.yaml: Bump actions/checkout from 5 to 6.

See: #36

Author: dependabot[bot]

.github/workflows/make-cross.yaml

@@ -50,7 +50,7 @@ jobs:
         run: lscpu
 
       - name: checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: install toolchain for ${{ matrix.arch }}
         run: |