Make `python` command fall back from system to asdf version

Hi. I have python 3.9 installed in system, and 3.8 installed in asdf.

I would like the `python` command to use version 3.9 and the `python3.8` command to use version 3.8.

I tried the following commands, but it did not work:

```
$ asdf global python system 3.8.6

$ python -V
Python 3.9.1

$ python3.8 -V
No python3.8 executable found for python system
```

Hope someone knows why the `python3.8` command doesn't work.

Reproduce the reported `asdf global python system 3.8.6`, `python -V`, and `python3.8 -V` commands first. Trace how the Python plugin resolves versioned executables when `system` is listed, and consider the work complete when `python` uses the system Python while `python3.8` resolves to the asdf-installed version.

Written by the indexing model from the issue text.