Is the salary received by an employee from a private company that inspects food, agricultural, and industrial materials, and whose work includes inspecting alcoholic beverages, lawful or unlawful? And does the same ruling apply to the salaries of customs officials who deal with alcoholic materials?

There is no blame upon you concerning the salary you receive, because your work is limited to maintaining and repairing the company's computers and installing networks. The fact that the company inspects alcoholic beverages does not make your salary unlawful. The company's funds are mixed, and dealing with someone whose money is mixed is disliked (makruh (definition)), not prohibited (haram (definition)). If you find another job free from such issues, then praise be to Allah. Otherwise, you are permitted to remain in your current job.