Which uses libc via CGO or parses /etc/passwd with no CGO, which won't work for some cases.

https://github.com/tailscale/tailscale/blob/e4144230f410204a...

  // userLookupGetent uses "getent" to look up users so that even with static
  // tailscaled binaries without cgo (as we distribute), we can still look up
  // PAM/NSS users which the standard library's os/user without cgo won't get
  // (because of no libc hooks). If "getent" fails, userLookupGetent falls back
  // to the standard library.