CI: Partially sync workflows and actions with 3.x branch

Stick to `ubuntu:20.04` runners for now, as porting to newer ones implies
fixing a number of compilation warnings which may not be worth the trouble
for an EOL branch.

core/core_builders.py

@@ -3,6 +3,7 @@
 All such functions are invoked in a subprocess on Windows to prevent build flakiness.
 
 """
+
 from platform_methods import subprocess_main
 from compat import iteritems, itervalues, open_utf8, escape_string, byte_to_str