Update action-download-artifact version to v6 for security (#442)

Author: filip-parity

.github/workflows/test-external-projects.yml

@@ -99,7 +99,7 @@ jobs:
 
       - name: Download master branch results
         if: github.ref != 'refs/heads/master'
-        uses: dawidd6/action-download-artifact@v3
+        uses: dawidd6/action-download-artifact@v6
         continue-on-error: true
         with:
           workflow: test-external-projects.yml