[css-mixins-1] Don't erase the result type (#12259)

The "result" registration got caught in the blast radius when resetting
the types for the local variables.

Co-authored-by: Anders Hartvoll Ruud <andruud@google.com>

Author: andruud

css-mixins-1/Overview.bs

@@ -589,7 +589,8 @@ with its [=function parameters=] overriding "inherited" custom properties of the
 		Let |argument styles| be the result.
 	10. Let |body rule| be the [=function body=] of |custom function|,
 		as a [=style rule=].
-	11. For each [=custom property registration=] of |registrations|,
+	11. For each [=custom property registration=] of |registrations|
+		except the registration with the name "result",
 		set its initial value
 		to the corresponding value in |argument styles|,
 		set its syntax