Adds check for no_install flag to check for reusing existing venv (wn…

…trblm#394)

reuse_existing is now true if we are not installing pacakges.

nox/sessions.py

@@ -536,7 +536,7 @@ def _create_venv(self) -> None:
             return
 
         reuse_existing = (
-            self.func.reuse_venv or self.global_config.reuse_existing_virtualenvs
+            self.func.reuse_venv or self.global_config.reuse_existing_virtualenvs or self.global_config.no_install
         )
 
         if backend is None or backend == "virtualenv":