buildman: Correct overwriting of settings file

The toolchain test causes the settings file to be overwritten, which is
annoying for local development. Fix it by passing None as the filename.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/buildman/toolchain.py

@@ -420,7 +420,7 @@ class Toolchains:
         Returns:
             Resolved string
 
-        >>> bsettings.Setup()
+        >>> bsettings.Setup(None)
         >>> tcs = Toolchains()
         >>> tcs.Add('fred', False)
         >>> var_dict = {'oblique' : 'OBLIQUE', 'first' : 'fi${second}rst', \