Backport fix for edit-vault-metadata.py

Use Python 3.8 instead of distribution default Python 3.6, because
Python 3.6 has different (incompatible) arguments for the subprocess
module.

tools/edit-vault-metadata.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/local/bin/python3
 
 """
    edit-vault-metadata : script for manually editing metadata of a data package