Expand Windows environment setup

- Write required environment variables to a batch file.
- Fix virtualenv bin path.
- Temporarily disable host_build and cargo steps on Windows.
- Update the python runner to work on Windows.
- Remove script_exectuable from the .gn file as it was added before
  Pigweed had an env_setup and was used to point to the correct system
  Python.

Change-Id: Ie4d0d0c5deb752c79ee5059dd812175775eae10c
diff --git a/.gn b/.gn
index de099cc..60ce6bd 100644
--- a/.gn
+++ b/.gn
@@ -1,4 +1,4 @@
-# Copyright 2019 The Pigweed Authors
+# Copyright 2020 The Pigweed Authors
 #
 # Licensed under the Apache License, Version 2.0 (the "License"); you may not
 # use this file except in compliance with the License. You may obtain a copy of
@@ -13,4 +13,3 @@
 # the License.
 
 buildconfig = "//BUILDCONFIG.gn"
-script_executable = "python3"