all: prepare for GOOS=ios in Go 1.16

Work with either way for now on iOS (darwin/arm64 vs ios/arm64).

In February when Go 1.16 comes out we'll have a universal binary for
darwin/arm64 (macOS) and will drop support for Go 1.15 and its
darwin/amd64 meaning iOS. (it'll mean macOS).

Context:

* https://tip.golang.org/doc/go1.16#darwin
* https://github.com/golang/go/issues/38485
* https://github.com/golang/go/issues/42100

ipn/local.go

@@ -1549,7 +1549,8 @@ func (b *LocalBackend) TestOnlyPublicKeys() (machineKey tailcfg.MachineKey, node
 // clients. We can't do that until 1.0.x is no longer supported.
 func temporarilySetMachineKeyInPersist() bool {
 	//lint:ignore S1008 for comments
-	if runtime.GOOS == "darwin" || runtime.GOOS == "android" {
+	switch runtime.GOOS {
+	case "darwin", "ios", "android":
 		// iOS, macOS, Android users can't downgrade anyway.
 		return false
 	}