build: Skip i686 build by default in gitian

Summary:
Backport of core PR18104.

From the PR conversation, it is made clear that nobody really uses the
shipped binary for 32 bits linux. We can safely stop distributing it as
part of our release process, but still make it possible to build from
source if desired. The build is now tested on CI to prevent from
regression.

Test Plan: Run the Gitian Linux build.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7229