[Chore] Update "actions/checkout"

Problem: node16 is now deprecated and github-runner provided by nixpkgs
no longer supports this runtime. However, "actions/checkout@v3" uses
this runtime.

Solution: Update CI pipeline to use "actions/checkout@v4".

Author: rvem

.github/workflows/check.yml

@@ -5,7 +5,7 @@ jobs:
   check:
     runs-on: self-hosted
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - name: check flake
         run: nix flake check -L