Bump actions/checkout from 2 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v2...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/build-sd-image.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Install QEMU
         run: sudo apt-get install qemu-user-static

.github/workflows/nix.yml

@@ -19,7 +19,7 @@ jobs:
           - harmonica
     runs-on: ubuntu-24.04
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: cachix/install-nix-action@v16
       # TODO: add a binary cache
       # - uses: cachix/cachix-action@v10
@@ -32,6 +32,6 @@ jobs:
   check-formatting:
     runs-on: ubuntu-24.04
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: cachix/install-nix-action@v16
       - run: nix shell nixpkgs#alejandra -c alejandra -c .