Remove osx64 from pixi, take 2 (#1777)

Supersedes #1768. That had conflicts and I wanted to start fresh since
`pixi update` takes 15 mins.

So this removes `osx-64` from our platforms, as nobody is using it. This
will make our lockfile smaller and hopefully a bit easier to resolve.
Then I ran `pixi update` to update all our dependencies and remove
osx-64.