Upgrade checkout action from v5 to v6

Author: galargh

.github/workflows/test.yml

@@ -35,7 +35,7 @@ jobs:
   #     RUST_LOG: debug
   #     RUSTFLAGS: -C target-cpu=native
   #   steps:
-  #     - uses: actions/checkout@v5
+  #     - uses: actions/checkout@v6
   #     - uses: ./.github/actions/setup-environment
   #       with:
   #         gpu: true
@@ -59,7 +59,7 @@ jobs:
       RUSTFLAGS: -C target-cpu=native
       RUST_TEST_TIMEOUT: 0
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: ./.github/actions/setup-environment
         with:
           gpu: true