Switch from setup-uv to an explicit uv bootstrap

[woodruffw]

Two reasons for doing this:

1. It should be a little faster.
2. It'll make it a bit easier to isolate this action from other actions/workflow runtime context, e.g. in the event that other actions run setup-uv and clobber the tool path.

xref pyca/cryptography#13991 for some more context.