hikey960: mkdtimg: force to use python2

to workaround the build error reported like this:

    FAILED: out/target/product/hikey960/dt.img
    /bin/bash -c "vendor/linaro//hikey960/20221126/bootloader/mkdtimg -c -d out/target/product/hikey960/hi3660-hikey960.dtb -o out/target/product/hikey960/dt.img"
    Traceback (most recent call last):
      File "/home/buildslave/srv/aosp-master/build/vendor/linaro//hikey960/20221126/bootloader/mkdtimg", line 159, in <module>
        main()
      File "/home/buildslave/srv/aosp-master/build/vendor/linaro//hikey960/20221126/bootloader/mkdtimg", line 153, in main
        dtimg_head_info = dt_head_info('HSDT', dtimg_version, dtb_count)
    TypeError: expected bytes, str found

Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
diff --git a/src/hikey960/bootloader/mkdtimg b/src/hikey960/bootloader/mkdtimg
index 6572204..33c982e 100755
--- a/src/hikey960/bootloader/mkdtimg
+++ b/src/hikey960/bootloader/mkdtimg
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 # Copyright 2017, The Android Open Source Project
 #
 # Licensed under the Apache License, Version 2.0 (the "License");