assets.py: use SNEJ_ROOT variable & os.path.join

scripts/assets.py

@@ -313,7 +313,9 @@ def stitch_sprites(metadata, filename_base):
 
 
 def main(args):
-  os.chdir(os.path.expanduser('~/snej/assets/time_fantasy'))
+  snej_root = os.environ.get('SNEJ_ROOT', os.path.join('~', 'snej'))
+  time_fantasy_path = os.path.join(snej_root, 'assets', 'time_fantasy')
+  os.chdir(os.path.expanduser(time_fantasy_path))
 
   sprite_files = unglob(SPRITE_FILES)
   tileset_files = unglob(TILESET_FILES)