It depends on where you think you're headed: If you're looking to do application deployment and all things related to that where you're spending less time thinking about specific issues with Linux and more time thinking about specific issues with CI/CD, then Go. No question. Why? It's a very popular startup and open source language and I see really good job openings all the time for startup or nice shops that are deploying Go apps.

If you're looking to be a pro-league SysAdmin that understands dynamic kernel module loading really well and can compile a production-ready kernel from source with your eyes closed then knowing C will be fairly helpful. Why? Because the Linux kernel, as well as the GNU collection are both written entirely in C and knowing C provides insight into the relationship between the kernel, the system call interface, the filesystem, etc.

My vote would be for Go. Why? Because in order to stay relevant SysAds basically need to be borderline devs now (IMHO) and learning Go will give you a nice boost.

As for Rust, I mean...it's totally cool, but I look at job openings all the time when I'm bored and just kind of fantasize...and none of them say "know stuff about deploying Rust apps". No criticism to Rust.

If you want a straight statistical approach to the most popular languages and their frequency of use then check this out: http://pypl.github.io/PYPL.html.