[CI] adapt to new upload/download-artifact changes

https://github.com/actions/upload-artifact?tab=readme-ov-file#breaking-changes

> Due to how Artifacts are created in this new version, it is no longer possible to upload to the same named Artifact multiple times. You must either split the uploads into multiple Artifacts with different names, or only upload once. Otherwise you will encounter an error.

This caused the PyPI upload job to fail earlier. Hopefully this should fix it.

.github/workflows/wheels.yml

@@ -34,6 +34,7 @@ jobs:
         python setup.py sdist bdist_wheel
     - uses: actions/upload-artifact@v4
       with:
+        name: pure
         path: |
           dist/*.whl
           dist/*.tar.gz
@@ -72,6 +73,7 @@ jobs:
         CIBW_ARCHS: ${{ matrix.arch }}
     - uses: actions/upload-artifact@v4
       with:
+        name: wheels
         path: wheelhouse/*.whl
 
   build_arch_wheels:
@@ -98,6 +100,7 @@ jobs:
         CIBW_ARCHS: ${{ matrix.arch }}
     - uses: actions/upload-artifact@v4
       with:
+        name: aarch64-wheels
         path: wheelhouse/*.whl
 
   deploy:
@@ -112,7 +115,8 @@ jobs:
     steps:
     - uses: actions/download-artifact@v4
       with:
-        name: artifact
+        # so that all artifacts are downloaded in the same directory specified by 'path'
+        merge-multiple: true
         path: dist
     - uses: pypa/gh-action-pypi-publish@v1.8.11
       with: