Set is_temporary.

ognibuild/session/__init__.py

@@ -100,6 +100,8 @@ class Session(object):
     def external_path(self, path: str) -> str:
         raise NotImplementedError
 
+    is_temporary: bool
+
 
 class SessionSetupFailure(Exception):
     """Session failed to be set up."""

ognibuild/session/plain.py

@@ -112,3 +112,5 @@ class PlainSession(Session):
 
     def setup_from_directory(self, path):
         return path, path
+
+    is_temporary = False

ognibuild/session/schroot.py

@@ -215,3 +215,5 @@ class SchrootSession(Session):
         export_directory = os.path.join(directory, subdir)
         shutil.copytree(path, export_directory, dirs_exist_ok=True)
         return export_directory, os.path.join(reldir, subdir)
+
+    is_temporary = True