scripts: Reverted del to resolve shadowed builtins

I don't know how I completely missed that this doesn't actually work!

Using del _does_ work in Python's repl, but it makes sense the repl may
differ from actual function execution in this case.

The problem is Python still thinks the relevant builtin is a local
variables after deletion, raising an UnboundLocalError instead of
performing a global lookup. In theory this would work if the variable
could be made global, but since global/nonlocal statements are lifted,
Python complains with "SyntaxError: name 'list' is parameter and
global".

And that's A-Ok! Intentionally shadowing language builtins already puts
this code deep into ugly hacks territory.

scripts/dbgflags.py

@@ -205,8 +205,9 @@ FLAGS = [
 def main(flags, *,
         list=False,
         all=False):
-    list_ = list; del list
-    all_ = all; del all
+    import builtins
+    list_, list = list, builtins.list
+    all_, all = all, builtins.all
 
     # first compile prefixes
     prefixes = {}