chore(ci): bump checkout to use non-deprecated Node version

Author: gekoke

.github/workflows/flake-check.yaml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Install Nix
         uses: DeterminateSystems/nix-installer-action@v1
       - name: Enable magic Nix cache