prober: allow monitoring of nodes marked as STUN only in default derpmap

prober uses NewRegionClient() to connect to a derper using a faked up
single-node region, but NewRegionClient() fails to connect if there is
no non-STUN only client in the region. Set the STUN only flag to false
before we call NewRegionClient() so we can monitor nodes marked as
STUN only in the default derpmap.

Updates #11492

Signed-off-by: Val <valerie@tailscale.com>

prober/derp.go

@@ -348,6 +348,8 @@ func newConn(ctx context.Context, dm *tailcfg.DERPMap, n *tailcfg.DERPNode) (*de
 	priv := key.NewNode()
 	dc := derphttp.NewRegionClient(priv, l, nil /* no netMon */, func() *tailcfg.DERPRegion {
 		rid := n.RegionID
+		// Allow the prober to monitor nodes marked as STUN only in the default map
+		n.STUNOnly = false
 		return &tailcfg.DERPRegion{
 			RegionID:   rid,
 			RegionCode: fmt.Sprintf("%s-%s", dm.Regions[rid].RegionCode, n.Name),