tox.ini: passenv TERM

Prevent having such issues locally:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/diffoscope/main.py", line 758, in main
    log_handler = ProgressManager().setup(parsed_args)
  File "/usr/lib/python3/dist-packages/diffoscope/progress.py", line 83, in setup
    bar = ProgressBar()
  File "/usr/lib/python3/dist-packages/diffoscope/progress.py", line 251, in __init__
    self.bar = OurProgressBar(
  File "/usr/lib/python3/dist-packages/diffoscope/progress.py", line 233, in __init__
    self.erase_to_eol = line_eraser(self.fd)
  File "/usr/lib/python3/dist-packages/diffoscope/logging.py", line 30, in line_eraser
    curses.setupterm(fd=fd.fileno())
_curses.error: setupterm: could not find terminfo database