buildman: Use env to pick the python from $PATH

python used in buildman doesn't need to be placed in
/usr/bin/python, So use env to ensure that the interpreter
will pick the python from environment.

Usefull with several versions of python's installed on system.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Acked-by: Simon Glass <sjg@chromium.org>