Add CI for a big-endian architecture (s390x)

[jamessan]

Running these jobs before the recent big-endian fix shows the failing test.

[kovidgoyal]

I'd prefer to stick to one CI provider, if at all possible. kitty uses github actions for CI.

[jamessan]

Understood. It's more complicated to test other architectures with Github Actions. I'll see what I can do.