chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yaml

@@ -15,7 +15,7 @@ jobs:
         os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: cachix/install-nix-action@v31
     - uses: cachix/cachix-action@v16
       with:
@@ -30,7 +30,7 @@ jobs:
         os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: cachix/install-nix-action@v31
     - uses: cachix/cachix-action@v16
       with:
@@ -49,7 +49,7 @@ jobs:
         os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: cachix/install-nix-action@v31
     - uses: cachix/cachix-action@v16
       with: