Fix the following two issues:
- pyenv root detection issue
When `PYENV_ROOT` environment variable is not set, neovim will detect
pyenv's root via `pyenv root` command, but which will be always fail
because `vim.fn.system()` returns result with additional `\n`. Using
`vim.system` instead prevents this problem. to trim it before check
whether it is exists
- python executable path detection issue
Filter unrelated `python-config` in cases where multiple python versions
are installed, e.g. `python-config`, `python3.10-config`,
`python3.11-config` etc.
b4b4cf46a7