Update actions/checkout action to v5 (#12)

Author: SuperSandroBot

.github/workflows/deadnix.yaml

@@ -14,7 +14,7 @@ jobs:
   deadnix:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
       - name: Install Nix
         uses: nixbuild/nix-quick-install-action@v30

.github/workflows/flake-check.yaml

@@ -20,7 +20,7 @@ jobs:
         target: [ "25.05", "unstable" ]
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
       - name: Install Nix
         uses: nixbuild/nix-quick-install-action@v30

.github/workflows/post-compare-link.yaml

@@ -12,7 +12,7 @@ jobs:
   post-compare-link:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
         with:
           fetch-depth: 0
       - uses: NuschtOS/flake-lock-compare-action@main

.github/workflows/template-sync.yaml

@@ -13,7 +13,7 @@ jobs:
     if: github.repository != 'NuschtOS/template'
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
         with:
           token: ${{ secrets.GH_TOKEN_FOR_UPDATES }}
       - uses: AndreasAugustin/actions-template-sync@v2