chore(deps): update actions/checkout action to v6 (#15)

Signed-off-by: bootc-bot[bot] <225049296+bootc-bot[bot]@users.noreply.github.com>
Co-authored-by: bootc-bot[bot] <225049296+bootc-bot[bot]@users.noreply.github.com>

Author: bootc-bot[bot]

.github/workflows/build.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Build and run CI in devcontainer
         uses: devcontainers/ci@v0.3
@@ -33,7 +33,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Run Kani proofs in devcontainer
         uses: devcontainers/ci@v0.3