feat&refactor: update AWS configuration guidelines and improve environment variable handling

desktop_env/providers/aws/AWS_GUIDELINE.md

@@ -26,18 +26,11 @@ You need to assign values to several variables crucial for the operation of thes
   - Example: `"osworld_key"`
 - **`NETWORK_INTERFACES`**: Configuration settings for network interfaces, which include subnet IDs, security group IDs, and public IP addressing.
   - Example:
-    ```python
-    NETWORK_INTERFACES = {
-        "us-east-1": [
-            {
-                "SubnetId": "subnet-037edfff66c2eb894",
-                "AssociatePublicIpAddress": True,
-                "DeviceIndex": 0,
-                "Groups": ["sg-0342574803206ee9c"]
-            }
-        ],
-        # Add configurations for other regions
-    }
+    ```bash
+    <!-- in .env file -->
+    AWS_REGION=us-east-1
+    AWS_SUBNET_ID=subnet-xxxx
+    AWS_SECURITY_GROUP_ID=sg-xxxx
     ```