Fix 6.1 build on i386 (and hopefully other arches)

[tianon]

Redmine 6.1 is failing to build on every architecture except amd64 and arm64, which isn't terribly unexpected (given those are the only ones that are likely to have published precompiled dependencies and #390 removed the variable that had us building all our dependencies on every architecture).

This adds i386 building to CI (in a commit by itself so we can see that the build currently fails), and a follow-up commit which makes i386 building succeed (with the implied hope that it fixes more architectures too).

[tianon]

Doh, bookworm, my dude:

error: package `clap v4.5.42` cannot be built because it requires rustc 1.74 or
newer, while the currently active rustc version is 1.63.0
Either upgrade to rustc 1.74 or newer, or use
cargo update -p clap@4.5.42 --precise ver
where `ver` is the latest version of `clap` supporting rustc 1.63.0

[tianon]

_{*quietly* what if we just didn't bookworm for 6.1}