Fix the use of multiple artifacts with same name

The v4 of upload-artifact/download-artifact no longer supports uploads
with the same name being extended, instead, we now need to create a new
upload for each run, and download each one.

see <https://github.com/actions/upload-artifact/blob/main/docs/MIGRATION.md#multiple-uploads-to-the-same-named-artifact>

.github/workflows/unit-tests.yml

@@ -44,7 +44,7 @@ jobs:
       - name: Upload coverage artifact
         uses: actions/upload-artifact@v4
         with:
-          name: coverage
+          name: coverage.${{ matrix.platform }}.${{ matrix.python-version }}
           path: .coverage.${{ matrix.platform }}.${{ matrix.python-version }}
           retention-days: 1
           if-no-files-found: error
@@ -64,10 +64,11 @@ jobs:
           python-version: 3.11
           install-args: "--no-root --only test"
 
-      - name: Download coverage artifact
+      - name: Download all coverage artifacts
         uses: actions/download-artifact@v4
         with:
-          name: coverage
+          pattern: coverage.*
+          merge-multiple: true # support downloading multiple artifacts to same dir
 
       # Combine all of the coverage files (for each os, python version - from matrix)
       # into a single coverage file (.coverage), and produce a final (combined) coverage report.