use python3 instead of python in build.sh

[topazus]

In some Linux distribution, the python executable is not symlink to python3 by default, such as Ubuntu, openSUSE. So it is more proper to change python to python3, which is ok under these conditions. test on Ubuntu:

$ ls -l /usr/bin/python*
lrwxrwxrwx 1 root root       9 Mar 13  2020 /usr/bin/python3 -> python3.8
-rwxr-xr-x 1 root root 5494584 Nov 14 12:59 /usr/bin/python3.8
lrwxrwxrwx 1 root root      33 Nov 14 12:59 /usr/bin/python3.8-config -> x86_64-linux-gnu-python3.8-config
lrwxrwxrwx 1 root root      16 Mar 13  2020 /usr/bin/python3-config -> python3.8-config

test on openSUSE:

$ ls -l /usr/bin/python*
lrwxrwxrwx 1 root root    10 Dec 13 02:15 /usr/bin/python3 -> python3.10
-rwxr-xr-x 1 root root 14448 Dec 13 02:15 /usr/bin/python3.10
-rwxr-xr-x 1 root root  3405 Dec 13 02:15 /usr/bin/python3.10-config
lrwxrwxrwx 1 root root    17 Dec 13 02:15 /usr/bin/python3-config -> python3.10-config