sb: Use shebang env python

Closes #4037

source-builder/sb/freebsd.py

@@ -27,9 +27,9 @@
 import pprint
 import os
 
-import check
-import error
-import execute
+from . import check
+from . import error
+from . import execute
 
 def load():
     uname = os.uname()