Use `rtx` instead of `pyenv`

[astrojuanlu]

Is your feature request related to a problem? Please describe

I'm always frustrated with pyenv shims. See for example the conversation in pyenv/pyenv#1112

Describe the behaviour you'd like

A runtime selector that does not use shims.

Describe alternatives you've considered

https://github.com/jdxcode/rtx

No shims - shims cause problems, they break which, and add overhead. By default, rtx does not use them—however you can if you want to.
...
Unlike asdf which uses shim files to dynamically locate runtimes when they're called, rtx modifies PATH ahead of time so the runtimes are called directly. This is not only faster since it avoids any overhead, but it also makes it so commands like which node work as expected. This also means there isn't any need to run asdf reshim after installing new runtime binaries.

Additional context

[mustard-mh]

Hey @astrojuanlu , thank you for your suggestions 🙏.

At the moment, we don't have any immediate plans to change our default workspace images. Making such a change would impact all of our users. However, if you feel strongly about this, we encourage you to ask other users to upvote 👍 this issue. This will help bring more attention to the topic and potentially influence our future decisions.

In the meantime, if you wish, you can explore the option of configuring a workspace image yourself by following the instructions provided in our documentation.

Let us know if you have any further questions or concerns.