Altered CI to upload coverage in one step

fixes https://github.com/TryGhost/Toolbox/issues/395

- we seem to be encountering issues with codecov where it randomly
  reports huge jumps in test coverage %
- I'm still not sure of the reason why, but the general trend in CI with
  codecov is to store test coverage files as artifacts and then submit
  them to codecov in one go
- this commit implements that in our CI
- let's see if this improves anything

.github/workflows/test.yml

@@ -61,8 +61,11 @@ jobs:
       # Merge coverage reports and upload
       - run: yarn ember coverage-merge
         working-directory: ghost/admin
-      - uses: codecov/codecov-action@v3
+      - uses: actions/upload-artifact@v3
         if: matrix.browser == 'Chrome'
+        with:
+          name: coverage
+          path: ghost/*/coverage/cobertura-coverage.xml
 
       - uses: daniellockyer/action-slack-build@master
         if: failure() && github.event_name == 'push' && github.ref == 'refs/heads/main'
@@ -163,8 +166,11 @@ jobs:
       - run: yarn
       - run: yarn workspaces run test:unit
 
-      - uses: codecov/codecov-action@v2
+      - uses: actions/upload-artifact@v3
         if: startsWith(matrix.node, '16')
+        with:
+          name: coverage
+          path: ghost/*/coverage/cobertura-coverage.xml
 
       - uses: daniellockyer/action-slack-build@master
         if: failure() && github.event_name == 'push' && github.ref == 'refs/heads/main'
@@ -342,6 +348,19 @@ jobs:
         env:
           SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}
 
+  coverage:
+    name: Coverage
+    needs: [unit-tests, admin-tests]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Restore coverage
+        uses: actions/download-artifact@v3
+        with:
+          path: ghost
+      - run: rsync -av --remove-source-files ghost/coverage/* ghost/
+      - uses: codecov/codecov-action@v3
+
   canary:
     needs: [lint, ghost-cli, admin-tests, migrations, unit-tests, database-tests]
     if: github.event_name == 'push' && github.ref == 'refs/heads/main'