wgengine/magicsock: make debugUseDerpRoute an opt.Bool.

Can still be constant, just needs the extra methods.

Fixes #3812

Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/magicsock/debugknobs_ios.go

@@ -4,13 +4,15 @@
 
 package magicsock
 
+import "tailscale.com/types/opt"
+
 // All knobs are disabled on iOS.
 // Further, they're const, so the toolchain can produce smaller binaries.
 const (
 	debugDisco                       = false
 	debugOmitLocalAddresses          = false
 	debugUseDerpRouteEnv             = ""
-	debugUseDerpRoute       = false
+	debugUseDerpRoute       opt.Bool = ""
 	logDerpVerbose                   = false
 	debugReSTUNStopOnIdle            = false
 	debugAlwaysDERP                  = false