Bump actions/checkout from 4 to 6 (#311)

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...v6)

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

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

Author: dependabot[bot]

.github/workflows/publish.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Setup Python
         uses: actions/setup-python@v6
         with:

.github/workflows/test.yml

@@ -30,7 +30,7 @@ jobs:
         mpi: ["nompi", "mpich", "openmpi"]
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Setup MPI
         if: ${{ matrix.mpi != 'nompi' }}
         uses: mpi4py/setup-mpi@v1
@@ -78,7 +78,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Setup Python ${{ matrix.python-version }}
         uses: conda-incubator/setup-miniconda@v3
         with:
@@ -124,7 +124,7 @@ jobs:
             lammps-version: "2024.08.29"
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Setup Python ${{ matrix.python-version }}
         uses: conda-incubator/setup-miniconda@v3
         with:
@@ -166,7 +166,7 @@ jobs:
         shell: bash -el {0}
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Setup Python 3.9
         uses: conda-incubator/setup-miniconda@v3
         with: