build failures due to inexplicable network errors

[Zack Weinberg <zackw@panix.com>]

"lb build" keeps failing with inexplicable network errors
("inexplicable" in the sense that I don't understand why on earth I
would get them):

W: Failed to fetch
http://ftp.us.debian.org/debian/dists/sid/Release.gpg  Unable to
connect to 127.0.0.1:http:
# I did not configure a proxy, why is it trying to contact 127.0.0.1?

Resolving ftp.us.debian.org (ftp.us.debian.org)... 2610:148:1f10:3::89
Connecting to ftp.us.debian.org
(ftp.us.debian.org)|2610:148:1f10:3::89|:80... failed: Network is
unreachable.
# The build machine has no IPv6 connectivity, so IPv6 has been disabled
# with sysctl, and no interface has an IPv6 address, so why is it trying to
# connect to an IPv6 address?

Plz help.

zw