pgflex: propagate environment to flex subprocess

pgflex: propagate environment to flex subprocess

Python's subprocess.run docs say that if the env argument is not None,
it will be used "instead of the default behavior of inheriting the
current process’ environment".  However, the environment should be
preserved, only adding FLEX_TMP_DIR to it.

Author: Javier Maestro <jjmaestro@ieee.org>
Discussion: https://www.postgresql.org/message-id/flat/CABvji06GUpmrTqqiCr6_F9vRL2-JUSVAh8ChgWa6k47FUCvYmA%40mail.gmail.com