Disable this test on Windows; it seems to not progress. (#37200)

I assume there's a common underlying problem for this and a lot of
other concurrency tests that are disabled on Windows.

Author: rjmccall

test/Concurrency/Runtime/custom_executors.swift

@@ -3,6 +3,7 @@
 // REQUIRES: concurrency
 // REQUIRES: executable_test
 
+// UNSUPPORTED: OS=windows-msvc
 // UNSUPPORTED: back_deployment_runtime
 
 actor Simple {