wscript: use consistent python shebang path

Most wscripts use
  #!/usr/bin/env python
Use that consistently.

The wscripts are not executed directly and do not need the shebang path,
but it might guide editors and other tools to recognize the files as
being Python-ish.

tools/bb/wscript

@@ -1,3 +1,4 @@
+#!/usr/bin/env python
 import waflib.Logs as Logs, waflib.Utils as Utils
 import os