The issue here is there is NO single system API for looking up user account entries on Linux.
It's implemented in libc. So you need to link to libc. Tailscale is a Go binary, and they probably prefer it to be statically-linked. glibc NSS implementation also REQUIRES you to load `.so` so you just can't emulate it in Go.
Then, "link to libc". Which libc? glibc? musl?