Add once() to migrate as done in upstream tasks

Noticing that the overridden `artisan:migrate` command is missing the upstream change to only have it run on the first server.

Was added in c1309f255fc6756c70c42465f76e3d08cbeebc7a

And re-added in deployphp/deployer#2169

src/task/artisan.php

@@ -9,7 +9,7 @@
 task('artisan:down', artisan('down', ['runInCurrent', 'showOutput']));
 
 desc('Execute artisan migrate');
-task('artisan:migrate', artisan('migrate --force', ['skipIfNoEnv']));
+task('artisan:migrate', artisan('migrate --force', ['skipIfNoEnv']))->once();
 
 desc('Execute artisan migrate:fresh');
 task('artisan:migrate:fresh', artisan('migrate:fresh --force'));