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 87bd16e)