buildman: Correct CROSS_COMPILE output for sandbox

At present, 'buildman -A sandbox' adds the path containing the
toolchain at present. We can assume that this is in the path and
we don't want to set CROSS_COMPILE=/bin/ so change this to align
with what MakeEnvironment() does.

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

tools/buildman/toolchain.py

@@ -156,9 +156,8 @@ class Toolchain:
         Returns:
             Value of that environment variable or arguments
         """
-        wrapper = self.GetWrapper()
         if which == VAR_CROSS_COMPILE:
-            return wrapper + os.path.join(self.path, self.cross)
+            return self.GetWrapper() + self.cross
         elif which == VAR_PATH:
             return self.path
         elif which == VAR_ARCH: