Bump actions/checkout from 5 to 6 in the all-actions group (#458)

Bumps the all-actions group with 1 update: [actions/checkout](https://github.com/actions/checkout).


Updates `actions/checkout` from 5 to 6
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: all-actions
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/pytest-snapshots.yaml

@@ -31,7 +31,7 @@ jobs:
         key: ubuntu-latest
 
     - name: Check out message-ix-models
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
         lfs: true
 

.github/workflows/pytest.yaml

@@ -71,7 +71,7 @@ jobs:
           ${{ secrets.MESSAGE_STATIC_DATA_PRIVATE_KEY }}
 
     - name: Check out message-static-data
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
         repository: iiasa/message-static-data
         path: message-static-data
@@ -229,7 +229,7 @@ jobs:
     needs: check
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with: { ref: "${{ needs.check.outputs.ref }}" }
     - uses: astral-sh/setup-uv@v7
       with: { python-version: "${{ env.python-version }}" }