Comment 4 for bug 1753340

launchpad-buildd sets http_proxy and https_proxy environment variables; in the current configuration they will contain a username and password, although this may not be true in future. Note that there's a unique proxy token for each build; you cannot expect to be able to hardcode it.

My experience here is that the hard bit (as in, I tried for several days and eventually gave up) is getting ant to cope with the username and password bits properly. That's why I proposed https://code.launchpad.net/~cjwatson/launchpad-buildd/local-snap-proxy/+merge/322545 to make that no longer necessary, but that's yet to have a full review.