Read default value for vardir option from env

Now the default value for the `vardir` option is taken from the `VARDIR`
env variable when it is defined.

lib/options.py

@@ -256,7 +256,7 @@ def __init__(self):
         parser.add_argument(
                 "--vardir",
                 dest="vardir",
-                default='/tmp/t',
+                default=os.environ.get('VARDIR') or '/tmp/t',
                 help=format_help(
                     """
                     Path to data directory.