Fix PyPi workflow to use newer `build` tool, and add back in checking out repo.

[mathsman5133]

If someone knows how to make the check box a compulsory "true" for the workflow_dispatch that would be good, too, but I'm not too fussed

[lukasthaler]

Nvm, that's an input. Something like inputs.confirmation should be available to check in a step

[mathsman5133]

Yep, do you know how that works (if the field exists / is empty) if it's called from an event (on release)? I'll have a look tomorrow otherwise

[lukasthaler]

I should be able to do that, but idk if I can get to it today

[mathsman5133]

OK, did a quick test and this seems to be good (skips when input is false, runs when true)... bit hard to test the release option but it should be fine :)