Update GitHub actions to their latest version

The behaviour of `actions/upload-artifact` have changed, we now need
distinct names for each artifact. This also means adjusting our use of
`actions/download-artifact` to collect all artifacts.

.github/workflows/tests.yml

@@ -17,9 +17,9 @@ jobs:
           python scripts/fetch-vendor.py --config-file scripts/ffmpeg-6.1.json /tmp/vendor
           PKG_CONFIG_PATH=/tmp/vendor/lib/pkgconfig python setup.py sdist
       - name: Upload source package
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
-          name: dist
+          name: dist-source
           path: dist/
 
   package-wheel:
@@ -73,19 +73,19 @@ jobs:
           cibuildwheel --output-dir dist
         shell: bash
       - name: Upload wheels
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
-          name: dist
+          name: dist-${{ matrix.os }}-${{ matrix.arch }}
           path: dist/
 
   publish:
     runs-on: ubuntu-latest
     needs: [package-source, package-wheel]
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/download-artifact@v3
+      - uses: actions/download-artifact@v4
         with:
-          name: dist
+          merge-multiple: true
           path: dist/
       - name: Publish to PyPI
         if: github.event_name == 'release' && github.event.action == 'published'