fix(health): check shell performance using vim.fn.system() #36980

Problem:
`vim.system()` doesn't invoke a shell, while the Vimscript function
`system()` does.

Solution:
Revert the change from 35af766de6 and use `vim.fn.system()` again to
actually invoke a shell.

runtime/lua/vim/health/health.lua

@@ -175,7 +175,8 @@ local function check_performance()
   -- check for slow shell invocation
   local slow_cmd_time = 1.5e9
   local start_time = vim.uv.hrtime()
-  system({ 'echo' })
+  -- Vimscript's sytem() is used to actually invoke a shell
+  vim.fn.system('echo')
   local elapsed_time = vim.uv.hrtime() - start_time
   if elapsed_time > slow_cmd_time then
     health.warn(