tsnet: fall back to 'tsnet' when os.Executable fails on darwin

Updates #19050

When tsnet.Server.start() is called with both Hostname and Dir explicitly
set, os.Executable() failure should not prevent the server from starting.
Extend the existing ios fallback to also cover darwin, where the same
failure occurs when the Go runtime is embedded in a framework launched
via Xcode's debug launcher.

Signed-off-by: Prakash Rudraraju <prakashrj@yahoo.com>
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