Support repo_dir in run configurations

[r4victor]

Currently, the repo dir is hardcoded to be /workflow. The proposal is to make it configurable with repo_dir with /workflow as a default for backward compatibility. Benefits

• /workflow is no longer hardcoded.
• A repo can be pre-cloned on a volume to avoid long pull for big repos.
• Support for use cases when /workflow cannot be used (conflict with OS image) or when other path is preferred.

Moreover, working_dir can default to repo_dir. Related to: #2003

Also consider introducing a run env variable that stores the path to the repo dir (e.g. DSTACK_REPO_DIR) so that the repo dir doesn't need to be hardcoded in run configurations