Add missing project SSH key to on-prem instances

[un-def]

Fixes: #1640

[r4victor]

It'd be nice to refactor the code to use project.ssh_private_key when connecting to remote instances such as here:

dstack/src/dstack/_internal/server/background/tasks/process_running_jobs.py

Lines 136 to 146 in 1ce1243

	server_ssh_private_key = project.ssh_private_key
	if (
	job_model.instance is not None
	and job_model.instance.remote_connection_info is not None
	and job_provisioning_data.dockerized
	):
	remote_conn_info: RemoteConnectionInfo = RemoteConnectionInfo.__response__.parse_raw(
	job_model.instance.remote_connection_info
	)
	server_ssh_private_key = remote_conn_info.ssh_keys[0].private

It may not be ideal doing in this release since it may break connecting to old instances. @un-def, maybe leave TODOs for that?

[jvstme]

This may also mean that other project users cannot use the on-prem instance for any runs but it needs to be verified.

@un-def, if this is still the case, please also add a new issue to fix it later by using the project key for remote instance connections from server and CLI.

[un-def]

@jvstme, it is not and was not the case, when a run is submitted to the shim, the user's public key (either provided by the user or generated by dstack), which is a part of the submitted task body, is dynamically added to authorized_keys.

The project key, as far as I understand, it's not even available to the user.