Bring back i386 CI

[narimiran]

This basically just reverts #17327, leaving bumping the Ubuntu version to 18.04 as the only change from the situation from before that PR.

This now finally makes it past the installing the dependencies part, and now reveals that json stuff (part of #17389, from what I can tell) is failing on 32-bit machines.

cc @timotheecour

[timotheecour]

reverting #17327 is not needed and not desirable; #17327 added useful things, eg:

• adds messages to the CI logs by automatically echoing each command before running it
• run bash sections with set -e so you catch errors early instead of silently ignoring them and then wondering why something unrelated failed later (which was the case at the time i wrote that PR)

the i386 issue was due to broken upstream packages that were broken for many days but the issue has been resolved, so simply un-disable linux i386 is the only thing needed, along with fixing the json tests for i386.

I'm doing that in #17469.