Bump actions/download-artifact from 5 to 6

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v5...v6)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  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/build.yml

@@ -318,7 +318,7 @@ jobs:
           python-version: ${{ matrix.python }}
 
       - name: Download cpymad wheels
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: ${{ matrix.os.dist }}
           path: dist
@@ -353,7 +353,7 @@ jobs:
       - uses: actions/setup-python@v6
         with:
           python-version: '3.9'
-      - uses: actions/download-artifact@v5
+      - uses: actions/download-artifact@v6
         with:
           name: dist-linux-manylinux2014_x86_64
           path: dist
@@ -387,7 +387,7 @@ jobs:
 
     steps:
       - name: Fetch built distributions
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           path: download