Bump actions/checkout from 4 to 7

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 7.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...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/cachix-upload.yml

@@ -33,7 +33,7 @@ jobs:
     runs-on: ${{ matrix.machine.os }}
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v7
         with:
           fetch-depth: 0
       - name: Setup Nix

.github/workflows/flakehub-publish.yaml

@@ -10,7 +10,7 @@ jobs:
       id-token: 'write'
       contents: 'read'
     steps:
-      - uses: 'actions/checkout@v4'
+      - uses: 'actions/checkout@v7'
       - uses: 'DeterminateSystems/nix-installer-action@v16'
       - uses: 'DeterminateSystems/flakehub-push@v5'
         with: