Connect to a specific machine when importing

We previously had a bug where, due to the fact that we weren't skipping
DNS registration, `fly pg import` would create a machine that tried to
import from itself, obviously failing. Although the DNS registration bug
is [being fixed](https://github.com/superfly/fly-proxy/pull/1192), I
still think that relying on skipping DNS registration is a little silly,
when we can just guarantee we'll connect to the correct machine using
6PN wizardry. `flyctl` needs to specify a machine ID to connect to
which, as of writing, is just any working machine in the app we want to
import into.

cmd/migrate/main.go

@@ -55,8 +55,16 @@ func main() {
 		os.Exit(1)
 		return
 	}
+	machineID := os.Getenv("PG_MACHINE_ID")
 
-	targetURI := fmt.Sprintf("postgres://postgres:%s@%s.internal:5432", operatorPass, appName)
+	appDomain := appName + ".internal"
+	if machineID != "" {
+		appDomain = machineID + ".vm." + appDomain
+	} else {
+		log.Println("[warn] PG_MACHINE_ID was not specified, falling back to resolving from app name")
+	}
+
+	targetURI := fmt.Sprintf("postgres://postgres:%s@%s:5432", operatorPass, appDomain)
 
 	opts := migrationOpts{
 		sourceURI: sourceURI,