update github action to v4

Author: piotrkwiecinski

Diff for: .github/workflows/test.yml

@@ -23,7 +23,7 @@ jobs:
           access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
           accept-flake-config = true
     - name: Checkout repo
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
     - name: Check emacs version
       run: nix develop .#${{ matrix.emacs_version }} -c emacs --version
     - name: Emacs support

Diff for: .github/workflows/update-snapshot.yml

@@ -9,7 +9,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         ssh-key: ${{ secrets.SSH_PRIVATE_KEY }}
     - uses: cachix/install-nix-action@v21