[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".

.github/workflows/main.yml

@@ -18,7 +18,7 @@ jobs:
  check:
  runs-on: [self-hosted, nix]
  steps:
- - uses: actions/checkout@v3
+ - uses: actions/checkout@v4
 
  - name: reuse lint
  run: nix shell .#packages.x86_64-linux.reuse -c reuse lint
@@ -47,7 +47,7 @@ jobs:
  runs-on: [self-hosted, nix]
  if: ${{ github.ref == 'refs/heads/master' }}
  steps:
- - uses: actions/checkout@v3
+ - uses: actions/checkout@v4
 
  - name: deploy webdemo
  run: |