cmd/tsidp,tsnet: update tsidp oidc-key store path (#16735)

The tsidp oidc-key.json ended up in the root directory
or home dir of the user process running it.

Update this to store it in a known location respecting
the TS_STATE_DIR and flagDir options.

Fixes #16734

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

tsnet/tsnet.go

@@ -1268,6 +1268,12 @@ func (s *Server) listen(network, addr string, lnOn listenOn) (net.Listener, erro
 	return ln, nil
 }
 
+// GetRootPath returns the root path of the tsnet server.
+// This is where the state file and other data is stored.
+func (s *Server) GetRootPath() string {
+	return s.rootPath
+}
+
 // CapturePcap can be called by the application code compiled with tsnet to save a pcap
 // of packets which the netstack within tsnet sees. This is expected to be useful during
 // debugging, probably not useful for production.