[NVIM] Make update checker portable

nvim/init.vim

@@ -96,7 +96,14 @@ local function check_plugin_updates()
     local concurrency = 2
 
     local function git_command(args)
-        local command = { 'ionice', '-c', '3', 'nice', '-n', '10', 'git' }
+        local command = {}
+        if vim.fn.executable('ionice') == 1 then
+            vim.list_extend(command, { 'ionice', '-c', '3' })
+        end
+        if vim.fn.executable('nice') == 1 then
+            vim.list_extend(command, { 'nice', '-n', '10' })
+        end
+        table.insert(command, 'git')
         vim.list_extend(command, args)
         return command
     end