fix(man.lua): :Man slow/hangs if MANPAGER is set #36689

Problem:
When `MANPAGER` is set to something like 'nvim +Man!',
`vim.system({ 'nvim' })` call waits forever for input and times out
after 10 seconds in `system()` and the assert on `stdout` being not
`nil` fails.

Solution:
Set `MANPAGER=cat` when calling `system()`

(cherry picked from commit 87bd16e470)

runtime/lua/man.lua

@@ -427,7 +427,8 @@ local function get_page(path, silent)
   if localfile_arg == nil then
     local mpath = get_path('man')
     -- Check for -l support.
-    localfile_arg = (mpath and system({ 'man', '-l', mpath }, true) or '') ~= ''
+    localfile_arg = (mpath and system({ 'man', '-l', mpath }, true, { MANPAGER = 'cat' }) or '')
+      ~= ''
   end
 
   local cmd = localfile_arg and { 'man', '-l', path } or { 'man', path }