Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/buildifier.yaml

@@ -14,6 +14,6 @@ jobs:
   check:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: buildifier
         run: bazel run --enable_bzlmod //.github/workflows:buildifier.check

.github/workflows/ci.yaml

@@ -26,7 +26,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - name: "Checkout the sources"
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Mount bazel caches
         uses: actions/cache@v4
         with:
@@ -51,7 +51,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - name: "Checkout the sources"
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Mount bazel caches
         uses: actions/cache@v4
         with: