Bump actions/checkout from 6 to 7

Bumps [actions/checkout](https://github.com/actions/checkout) from 6 to 7.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v6...v7)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '7'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

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

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@v7
 
       - name: Install QEMU
         run: sudo apt-get install qemu-user-static

.github/workflows/nix.yml

@@ -19,13 +19,13 @@ jobs:
           - harmonica
     runs-on: ubuntu-24.04
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
       - uses: cachix/install-nix-action@v31
       - run: nix flake check --accept-flake-config
       - run: nix build --accept-flake-config --dry-run .#nixosConfigurations.${{ matrix.target-system }}.config.system.build.toplevel
   check-formatting:
     runs-on: ubuntu-24.04
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
       - uses: cachix/install-nix-action@v31
       - run: nix flake check --accept-flake-config

.github/workflows/test-my-packages.yml

@@ -23,7 +23,7 @@ jobs:
           # - python311Packages.fastmri # broken for now
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@v7
 
       - name: Install Nix
         uses: cachix/install-nix-action@v31