pyenv: only run if pyenv not in $PATH (#8462)

This is needed if in between the update of `$PATH` the `$commands` parameter hasn't
been updated, i.e. if there was no `rehash` or `hash -rf` or automatic rehash.
Co-authored-by: Marc Cornellà <marc.cornella@live.com>

plugins/pyenv/pyenv.plugin.zsh

 # This plugin loads pyenv into the current shell and provides prompt info via
 # the 'pyenv_prompt_info' function. Also loads pyenv-virtualenv if available.
 
-FOUND_PYENV=$+commands[pyenv]
+# Load pyenv only if command not already available
+command -v pyenv &> /dev/null && FOUND_PYENV=1 || FOUND_PYENV=0
 
 if [[ $FOUND_PYENV -ne 1 ]]; then
     pyenvdirs=("$HOME/.pyenv" "/usr/local/pyenv" "/opt/pyenv" "/usr/local/opt/pyenv")