[Bug]: Cannot create cloud fleets with `placement: cluster`

[r4victor]

Steps to reproduce

1. Create a cluster fleet:

type: fleet
name: my-cluster-fleet
nodes: 2
placement: cluster
backends: [aws]

2. Observe that only the master node is provisioned. Other nodes fail with Offers not found.

Actual behaviour

This was introduced in #2266 and affects 0.18.41.

It does not affect implicitly created multi-node task fleets (without fleet configuration).

Expected behaviour

No response

dstack version

master

Server logs

Additional information

No response