oss_scripts will use python2 not python3

[zyxkad]

I want to use tensorflow-text on python 3, but i checked the build scripts, and it include the following lines

if (which python) | grep -q "python"; then
  installed_python="python"
elif (which python3) | grep -q "python3"; then
  installed_python="python3"
fi

It will always use python (in my environment, it will be linked to python2), but I want build it to python3, as same as how the tensorflow work here.

Another problem is that the build script will use hardcoded pip, which is different from the python it will use. I think it should use $installed_python -m pip to be better.

[EricLeFort]

This should take care of the Python version prioritization issue: 7c1b820

Will let the TF team get back to you on the second bit.