Updated build-virtualenv to use Python 3 features.
Updated build-virtualenv to use Python 3 features.

With Python 3, virtualenv is built-in. We thus remove the error
messages if the script is run using Python 3 without virtualenv
installed on the machine. We also use `python` instead of
`virtualenv` to build the virtual environment.

This change also fixes an issue on my computer where running
`build-virtualenv` with Python 3 displayed an error message:
`ModuleNotFoundError: No module named 'nodeenv'`.

Review: https://reviews.apache.org/r/67853/

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