dstack run fails after dstack init is called without proper credentials #175

Steps to reproduce:

1. Create a private repo
2. Run dstack init without any credentials.
3. Run a simple workflow:

workflows:
  - name: hello
    provider: bash
    commands:
      - echo Done

It will exit silently with status as failed.

The issue is fixied if you supply the credentials as dstack init -t my_github_token.

dstack init should check if the credentials are set up properly. At the moment, it performs the check by running git ls-remote but it's succesfull because git takes the token from gitcredentials – e.g. keychain on MacOS.