Add clone on single repo to action

[sverhoeven]

At the moment for a checking the current repo it is expected the workflow clones the repo with uses: actions/checkout@v2 step before running the tortellini action.

However the action can already clone repos when given a csv file with repo urls. It would be nice if it could also clone the active repo. Using env var to determine github url.

Pro:

• No need for uses: actions/checkout@v2 step,
• Both modes look very similar
  Con:
• actions/checkout does a good, job no need to replicate
• more maintenance

[sverhoeven]

Came up in #136 (comment)