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/.gitignore b/.gitignore
index 30aeb32..b9b2f70 100644
--- a/.gitignore
+++ b/.gitignore
@@ -43,5 +43,6 @@
 
 # Env Setup
 env_setup/.setup.sh
+env_setup/.env_setup.bat
 .cipd
 .cargo