Comment 3 for bug 1920033

Searching led me to:
https://askubuntu.com/questions/764610/how-to-install-snap-packages-behind-web-proxy-on-ubuntu-16-04

which seems to say it isn't 'snap set core proxy.http' but:
sudo snap set system proxy.http="http://<proxy_addr>:<proxy_port>"

Which was new in snap 2.28.
It may be we need to switch to 'system' since there are now more than just 'core' and 'core18'.