make installer use initial INVOKEAI_ROOT as default install location

installer/lib/main.py

@@ -3,6 +3,7 @@ InvokeAI Installer
 """
 
 import argparse
+import os
 from pathlib import Path
 from installer import Installer
 
@@ -15,7 +16,7 @@ if __name__ == "__main__":
         dest="root",
         type=str,
         help="Destination path for installation",
-        default="~/invokeai",
+        default=os.environ.get('INVOKEAI_ROOT') or "~/invokeai",
     )
     parser.add_argument(
         "-y",