runtime: don't execute external commands when loading ftplugins

This is a followup to 816fbcc262687b81fc46f82f7bbeb1453addfe0c (patch
9.0.1833: [security] runtime file fixes)

It basically disables that external commands are run on loading of the
filetype plugin, **unless** the user has set the `g:plugin_exec = 1`
global variable in their configuration or for a specific filetype the
variable g:<filetype>_exec=1.

There are a few more plugins, that may execute system commands like
debchangelog, gitcommit, sh, racket, zsh, ps1 but those do at least
do not run those commands by default during loading of the filetype plugin
(there the command is mostly run as convenience for auto-completion or
to provide documentation lookup).

closes: #13034

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Tim Pope <vim@tpope.org>
diff --git a/runtime/ftplugin/ruby.vim b/runtime/ftplugin/ruby.vim
index a424801..b61c176 100644
--- a/runtime/ftplugin/ruby.vim
+++ b/runtime/ftplugin/ruby.vim
@@ -61,6 +61,10 @@
 endif
 
 function! s:query_path(root) abort
+  " Disabled by default for security reasons.  
+  if !get(g:, 'ruby_exec', get(g:, 'plugin_exec', 0))
+    return []
+  endif
   let code = "print $:.join %q{,}"
   if &shell =~# 'sh' && empty(&shellxquote)
     let prefix = 'env PATH='.shellescape($PATH).' '
@@ -84,7 +88,7 @@
     else
       let path = split(system(path_check),',')
     endif
-    unlet s:tmp_cwd
+    unlet! s:tmp_cwd
     exe cd cwd
     return path
   finally