So non-local users (e.g. Kerberos on FreeIPA) on Linux can be looked
up. Our default binaries are built with pure Go os/user which only
supports the classic /etc/passwd and not any libc-hooked lookups.
Updates #12601
Change-Id: I9592db89e6ca58bf972f2dcee7a35fbf44608a4f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>