Build: Bump actions/download-artifact from 7 to 8

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 7 to 8.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v7...v8)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: '8'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/nightly-pypi-build.yml

@@ -74,7 +74,7 @@ jobs:
 
     steps:
     - name: Download all the artifacts
-      uses: actions/download-artifact@v7
+      uses: actions/download-artifact@v8
       with:
         merge-multiple: true
         path: dist/

.github/workflows/python-ci.yml

@@ -197,7 +197,7 @@ jobs:
     - name: Install dependencies
       run: uv sync --group dev
     - name: Download all coverage artifacts
-      uses: actions/download-artifact@v7
+      uses: actions/download-artifact@v8
       with:
         pattern: coverage-*
         merge-multiple: true