Module search path incorrectly includes system python installation

```
$ asdf install python 3.8.14
python-build 3.8.14 /home/sirianni/.asdf/installs/python/3.8.14
Downloading Python-3.8.14.tar.xz...
-> https://www.python.org/ftp/python/3.8.14/Python-3.8.14.tar.xz
Installing Python-3.8.14...
Installed Python-3.8.14 to /home/sirianni/.asdf/installs/python/3.8.14
```
```
$ /home/sirianni/.asdf/installs/python/3.8.14/bin/python -c 'import sys; print("\n".join(sys.path));'

/home/sirianni/.asdf/installs/python/3.8.14/lib/python38.zip
/home/sirianni/.asdf/installs/python/3.8.14/lib/python3.8
/home/sirianni/.asdf/installs/python/3.8.14/lib/python3.8/lib-dynload
/home/sirianni/.local/lib/python3.8/site-packages
/home/sirianni/.asdf/installs/python/3.8.14/lib/python3.8/site-packages
```

Note that `sys.path` incorrectly includes `/home/sirianni/.local/lib/python3.8/site-packages` which is from the system python installation.

Reproduce the reported behavior with the shown asdf install command and Python sys.path check. Trace how the asdf-python plugin configures the installed interpreter and verify the completed change no longer includes /home/sirianni/.local/lib/python3.8/site-packages in sys.path.

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