codecov: set patch check target to 0%

Hopefully this will make the patch builder merely informative and not reject any change without unit tests.

.github/codecov.yml

@@ -5,7 +5,7 @@ coverage:
         threshold: 0.1
     patch:
       default:
-        threshold: 0.1
+        target: 0%
 codecov:
   require_ci_to_pass: true
   notify: