Return control to pyenv-uninstall in uninstall/envs.bash

[aiguofer]

This would fix #320, since returning control to pyenv-uninstall would allow the rest of the before_uninstall and after_install hooks to run.

From my understanding, the exec would pass control to the other script and never return it, so it would never hit the exit.. and if it did, there's no real need to since they are in mutually exclusive if statements. However, I might be misunderstanding something here, or there might have been a reason it was done like that, so it's quite possible there's a problem with this solution that I'm not aware of.

One problem I noticed was the error when returning control to pyenv-uninstall. Setting FORCE=t avoids this error, although I'm not sure what exactly that does so I'm not sure if there's any issues with that.