build: update actions/upload-artifact and actions/download-artifact (#1910)

* build: update actions/upload-artifact and actions/download-artifact

* cater for breaking changes in v4

Author: parthea

synthtool/gcp/templates/python_library/.github/workflows/unittest.yml

@@ -26,9 +26,9 @@ jobs:
       run: |
         nox -s unit-${{ '{{' }} matrix.python {{ '}}' }}
     - name: Upload coverage results
-      uses: actions/upload-artifact@v3
+      uses: actions/upload-artifact@v4
       with:
-        name: coverage-artifacts
+        name: coverage-artifact-${{ '{{' }} matrix.python {{ '}}' }}
         path: .coverage-${{ '{{' }} matrix.python {{ '}}' }}
 
   cover:
@@ -47,11 +47,11 @@ jobs:
         python -m pip install --upgrade setuptools pip wheel
         python -m pip install coverage
     - name: Download coverage results
-      uses: actions/download-artifact@v3
+      uses: actions/download-artifact@v4
       with:
-        name: coverage-artifacts
         path: .coverage-results/
     - name: Report coverage results
       run: |
-        coverage combine .coverage-results/.coverage*
+        find .coverage-results -type f -name '*.zip' -exec unzip {} \;
+        coverage combine .coverage-results/**/.coverage*
         coverage report --show-missing --fail-under={{ cov_level if cov_level != None else 100 }}