Reuse existing virtualenv when building command line parser for docs

[jgraham]

This fixes building docs in the docker container.

[DanielRyanSmith]

Tangentially related as I'm seeing the same failures in your CI run,

Your CI failure seems to be related to the timeout set for checking if the webtransport HTTP/3 server is running here:

wpt/tools/wptrunner/wptrunner/environment.py

Lines 284 to 286 in 6827719

	if not webtranport_h3_server_is_running(host, port, timeout=1.0):
	# TODO(bashi): Consider supporting retry.
	failed.append((host, port))

I've been testing for Python 3.10 compatibility and for some reason this check times out much more regularly on Py3.10, which had me very confused and thinking it was a compatibility issue. Increasing this timeout eliminated this failure (for testing I increased to 5.0). It's possible we just need to increase the timeout here so these occasional unrelated failures don't happen.