fix some bugs

- fix tests when `--build` is set
- don't leak `config.example.toml` fd
- don't crash if `config.toml` doesn't exist yet

src/bootstrap/configure.py

@@ -400,7 +400,9 @@ def parse_example_config(known_args, config):
     targets = {}
     top_level_keys = []
 
-    for line in open(rust_dir + '/config.example.toml').read().split("\n"):
+    with open(rust_dir + '/config.example.toml') as example_config:
+        example_lines = example_config.read().split("\n")
+    for line in example_lines:
         if cur_section is None:
             if line.count('=') == 1:
                 top_level_key = line.split('=')[0]