Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/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
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yaml

@@ -15,7 +15,7 @@ jobs:
         # Login shell is required to include changes by conda init bash.
         shell: bash -l -eo pipefail {0}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: conda-incubator/setup-miniconda@v3
       with:
         python-version: "3.9"

.github/workflows/deploy-viz-app.yaml

@@ -26,7 +26,7 @@ jobs:
       url: ${{ steps.deployment.outputs.page_url }}
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: actions/setup-node@v6
         with:
           node-version: 18

.github/workflows/run-usa-models.yaml

@@ -38,7 +38,7 @@ jobs:
         with:
           name: model-outputs
       - name: Clone fork of variant nowcast hub
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           repository: nextstrain/variant-nowcast-hub
           path: variant-nowcast-hub