runtime(sh,zsh): clear $MANPAGER in ftplugin before shelling out

Say you use Vim and set MANPAGER='vim -M +MANPAGER --not-a-term -'; then
:{Zs,S}hKeywordPrg (or K) will crap out and spew terminal garbage into
less when bash's "help" fails. This was introduced by 2f25e40b1
(runtime: configure keywordpg for some file types (#5566), 2023-08-23)
and may be present in other files touched by that commit.

Make the "man" invocation sensible by unsetting MANPAGER in the
environment.

Note that changing MANPAGER for `:terminal` is not needed; Vim within
Vim is perfectly fine.

closes: #14679

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/ftplugin/zsh.vim b/runtime/ftplugin/zsh.vim
index 40986fc..aee8900 100644
--- a/runtime/ftplugin/zsh.vim
+++ b/runtime/ftplugin/zsh.vim
@@ -2,7 +2,7 @@
 " Language:             Zsh shell script
 " Maintainer:           Christian Brabandt <cb@256bit.org>
 " Previous Maintainer:  Nikolai Weibull <now@bitwi.se>
-" Latest Revision:      2023-10-07
+" Latest Revision:      2024 May 06 by Vim Project (MANPAGER=)
 " License:              Vim (see :h license)
 " Repository:           https://github.com/chrisbra/vim-zsh
 
@@ -24,7 +24,7 @@
   elseif has('terminal')
     command! -buffer -nargs=1 ZshKeywordPrg silent exe ':term zsh -c "autoload -Uz run-help; run-help <args>"'
   else
-    command! -buffer -nargs=1 ZshKeywordPrg echo system('zsh -c "autoload -Uz run-help; run-help <args> 2>/dev/null"')
+    command! -buffer -nargs=1 ZshKeywordPrg echo system('MANPAGER= zsh -c "autoload -Uz run-help; run-help <args> 2>/dev/null"')
   endif
   if !exists('current_compiler')
     compiler zsh