add logic for finding the root (runtime) directory (#1948)

This commit fixes the root search logic to be as follows:

1) The `--root_dir` command line argument
2) The contents of environment variable INVOKEAI_ROOT
3) The VIRTUAL_ENV environment variable, plus '..'
4) $HOME/invokeai

(3) is the new feature. Since we are now recommending to install
InvokeAI and its dependencies into the .venv in the root directory,
this should be a reliable choice.

ldm/invoke/globals.py

@@ -11,12 +11,13 @@
 '''
 
 import os
+import os.path as osp
 from argparse import Namespace
 
 Globals = Namespace()
 
 # This is usually overwritten by the command line and/or environment variables
-Globals.root = os.path.abspath(os.environ.get('INVOKEAI_ROOT') or os.path.expanduser('~/invokeai'))
+Globals.root = osp.abspath(os.environ.get('INVOKEAI_ROOT') or osp.abspath(osp.join(os.environ.get('VIRTUAL_ENV'),'..')) or osp.expanduser('~/invokeai'))
 
 # Where to look for the initialization file
 Globals.initfile = 'invokeai.init'