Fix migration step: run node as ubuntu user for correct PATH

SSM runs as root with minimal PATH. Use sudo -iu ubuntu to get the
ubuntu user's login shell environment where node is available.

.gitea/workflows/deploy.yml

@@ -160,7 +160,7 @@ jobs:
               "rm -rf repo.tar.gz filamenteka",
               "echo Migration files:",
               "ls -la /home/ubuntu/filamenteka-api/database/migrations/",
-              "export NVM_DIR=/home/ubuntu/.nvm && . $NVM_DIR/nvm.sh && cd /home/ubuntu/filamenteka-api && node api/migrate.js"
+              "sudo -iu ubuntu bash -c 'cd /home/ubuntu/filamenteka-api && node api/migrate.js'"
             ]' \
             --output json \
             --query "Command.CommandId" --output text)