Bump actions/checkout from 4 to 6

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] <support@github.com>

Author: dependabot[bot]

.github/workflows/build-spack-nodeps.yml

@@ -26,12 +26,12 @@ jobs:
     permissions:
       packages: write
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Update system software
         run: |
           bash .github/workflows/install_os_deps.sh
       - name: Clone Spack
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           # Take the latest (develop) version.
           repository: spack/spack

.github/workflows/pre-commit.yml

@@ -11,6 +11,6 @@ jobs:
   pre-commit:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: actions/setup-python@v5
       - uses: pre-commit/action@v3.0.1