[ffigen] Run ffigen CI less often on Apple silicon

[liamappelbe]

The mac arm64 bot isn't free, so we'll just run it once a week, rather than on every PR. CPU specific bugs don't come up very often, so this should be sufficient.

Fixes #845

[coveralls]

coverage: 92.672%. remained the same
when pulling a1d700c on ffigenci
into 5aeab29 on main.

[dcharkes]

Thanks @liamappelbe !