Clone URI defaults to HTTPS when an SSH private key is set

[silverlyra]

Describe the bug
The uri in the source configuration defaults to https://github.com/…, even when private_key is set.

To Reproduce

resource_types:
- name: pull-request
  type: docker-image
  source:
    repository: jtarchie/pr
resources:
- name: pr
  type: pull-request
  source:
    access_token: ((github-token))
    private_key: ((deploy-key))
    repo: …

Expected behavior
uri should default to git@github.com/(repo).git