Only include vim-pyenv plugin when pyenv is installed

lua/conf/plugins.lua

@@ -70,6 +70,8 @@ Plug 'sheerun/vim-polyglot'
 -------------------------
 
 -- Pyenv Support
-Plug 'lambdalisue/vim-pyenv'
+if vim.fn.executable('pyenv') == 1 then
+  Plug 'lambdalisue/vim-pyenv'
+end
 
 vim.call('plug#end')