Allocation with ephemeral_disk is garbage collected before migration to new node is completed

### Nomad version
1.5.2

### Operating system and Environment details
Ubuntu 20.04

### Issue
Setup:
Nomad client node uses default setting `gc_disk_usage_threshold = 80`
Client node disk is used up to 85%

We have a group with ephemeral_disk:
```
group "example" {
ephemeral_disk {
migrate = true
sticky = true
}
}
```

after draining a node on which the job is running, it fails to reallocate disk to new node with `error making snapshot`
bacuse garbage collection manages to be faster than the reallocation.
Shouldn't the gc process wait until the reallocation is completed?

### Reproduction steps
- Fill disk higher than `gc_disk_usage_threshold`
- Craete job that uses ephemeral disk
- Drain node on which the job is running

#### Expected Result
`ephemeral_disk` is moved firtst, then the allocation is garbage collected

#### Actual Result
allocation is garbage collected before it manages to move the `ephemeral_disk`

### Job file (if appropriate)

### Nomad Server logs (if appropriate)

### Nomad Client logs (if appropriate)
```
2023-03-29T10:13:55.115+0200 [ERROR] http: request failed: method=GET path=/v1/client/allocation/ad482d74-9008-7ef0-3799-ca523dbf83fd/snapshot error="error making snapshot: failed to snapshot /opt/nomad/alloc/ad482d74-9008-7ef0-3799-ca523dbf83fd/alloc/data: lstat /opt/nomad/alloc/ad482d74-9008-7ef0-3799-ca523dbf83fd/alloc/data: no such file or directory" code=500
```

Reproduce the issue on Nomad 1.5.2 by exceeding gc_disk_usage_threshold, creating a job with migrate and sticky ephemeral_disk settings, and draining its node. Start with the allocation snapshot request at /v1/client/allocation/{id}/snapshot and the reported error logs. Done means the ephemeral disk migration completes before garbage collection removes the allocation.

Written by the indexing model from the issue text.