Bump actions/checkout from 5 to 6 in the all-actions group

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]>

Author: dependabot[bot]

.github/workflows/nightly.yaml

@@ -34,7 +34,7 @@ jobs:
 
     steps:
     - name: Check out message_ix
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
         fetch-depth: ${{ env.depth }}
 

.github/workflows/pytest.yaml

@@ -82,7 +82,7 @@ jobs:
 
     steps:
     - name: Check out message_ix
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
         fetch-depth: ${{ env.depth }}
         ref: ${{ github.event_name != 'pull_request_target' && github.ref || github.event.pull_request.head.sha }}
@@ -158,7 +158,7 @@ jobs:
 
     steps:
     - name: Check out message_ix
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
         fetch-depth: ${{ env.depth }}
         ref: ${{ github.event_name != 'pull_request_target' && github.ref || github.event.pull_request.head.sha }}
@@ -230,7 +230,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         ref: ${{ github.event_name != 'pull_request_target' && github.ref || github.event.pull_request.head.sha }}
     - uses: astral-sh/setup-uv@v7