sandbox.py: Use _root in _has_command

This will need more attention when we bring in another virtual
directory backend, however, we've said it is acceptable for the
sandbox itself to access the underlying directory, and this is
the best fix in the meantime.

buildstream/sandbox/sandbox.py

@@ -316,11 +316,11 @@ class Sandbox():
     def _has_command(self, command, env=None):
         if os.path.isabs(command):
             return os.path.exists(os.path.join(
-                self.get_directory(), command.lstrip(os.sep)))
+                self._root, command.lstrip(os.sep)))
 
         for path in env.get('PATH').split(':'):
             if os.path.exists(os.path.join(
-                    self.get_directory(), path.lstrip(os.sep), command)):
+                    self._root, path.lstrip(os.sep), command)):
                 return True
 
         return False