Fix volume attachment with dstack backend

[un-def]

Fixes: #2172
See: #1681
See: #1682

[jvstme]

Isn't the same change needed in get_job_mount_point_volume? Otherwise, I think, the volumes will be attached to the instance but not mounted

[un-def]

@jvstme good catch!

It turned out that JobProvisioningData in process_submitted_jobs (the one I fixed) and in process_running_jobs are not the same, we have additional logic for running jobs:

dstack/src/dstack/_internal/server/services/jobs/__init__.py

Lines 98 to 103 in acf60c9

	if (
	job_provisioning_data.backend == BackendType.DSTACK
	and job_provisioning_data.backend_data is not None
	):
	backend_data = json.loads(job_provisioning_data.backend_data)
	job_provisioning_data.backend = backend_data["base_backend"]

That means that it "just" works without the additional fix your suggested, but it should be fixed indeed.

I think we no longer need this magic (since #1682), but I'll keep in for now with a comment.