all: don't rebind variables in for loops

See https://tip.golang.org/wiki/LoopvarExperiment#does-this-mean-i-dont-have-to-write-x--x-in-my-loops-anymore

Updates https://github.com/tailscale/tailscale/issues/11058

Signed-off-by: Alex Chan <alexc@tailscale.com>

wgengine/magicsock/derp.go

@@ -836,7 +836,6 @@ func (c *Conn) maybeCloseDERPsOnRebind(okayLocalIPs []netip.Prefix) {
 			c.closeOrReconnectDERPLocked(regionID, "rebind-default-route-change")
 			continue
 		}
-		regionID := regionID
 		dc := ad.c
 		go func() {
 			ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)