This fixes "tailscale cert" on Synology where the var directory is
typically like /volume2/@appdata/Tailscale, or any other tailscaled
user who specifies a non-standard state file location.
This is a interim fix on the way to #2932.
Fixes#2927
Updates #2932
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>