Use nightly pandas builds again

[honno]

Until #11 we were happily using nightly builds from here, but I was facing some unexpected problems in terms of what pandas versions where actually being installed (even on clean environments). So I just build pandas from source on CI for now, but that's awfully slow and so makes CI not particularly ergonomic. I need to investigate if I'm doing something wrong or can find a hacky solution (can get a main-branch pandas with --pre, but that wasn't interacting nicely with --no-deps/--ignore-installed).

[honno]

Fortunately pandas had its 1.5 release recently, which includes all the recent changes to pandas.api.interchange that our tests/wrappers.py assumed. The workflow has been updated in #14.

It would still be ideal to get nightly builds working again to catch errors. I had another look at the aforementioned nightly builds to no success, so need to have another go at some point—if there's an issue with the packaging, I need to raise an issue or email someone heh.