trimmed builds don't have absolute path information in executable
metadata, which leads the runtime.Caller approach failing
mysteriously in yarn with complaints about relative package paths.
So, instead of using embedded package metadata to find paths,
expect that we're being invoked within the tailscale repo, and
locate the tsconnect directory that way.
Signed-off-by: David Anderson <danderson@tailscale.com>
0b8f89c79c