fix(checkhealth): skip vim.health #18816

Problem:
https://github.com/neovim/neovim/pull/18720#issuecomment-1142614996

The vim.health module is detected as a healthcheck, which produces spurious errors:

    vim: require("vim.health").check()
    ========================================================================
    - ERROR: Failed to run healthcheck for "vim" plugin. Exception:
      function health#check, line 20
      Vim(eval):E5108: Error executing lua [string "luaeval()"]:1: attempt to call field 'check' (a nil value)
      stack traceback:
      [string "luaeval()"]:1: in main chunk

Solution:
Skip vim.health when discovering healthchecks.

runtime/lua/vim/health.lua

@@ -41,6 +41,8 @@ M._complete = function()
   vim.tbl_map(function(f)
     unique[f] = true
   end, names)
+  -- vim.health is this file, which is not a healthcheck
+  unique['vim'] = nil
   return vim.tbl_keys(unique)
 end