jenkins-rel: Configure proxy environment variables at node level

Intermittent proxy disconnect were causing failures during branch.py's
polling of the pushed mediawiki/core change in Gerrit. This is avoided
by configuring NO_PROXY.

However, instead of configuring this at the job level, it makes more
sense to ensure all jobs running on the releases node use the correct
proxy settings. Configure http_proxy, https_proxy, and no_proxy at
the node level and use the shortened/relative webproxy hostname as our
hosts are all configured with the right search domain.

Bug: T374138