Switch supported actions/checkout from v1 to v2

[SeanTAllen]

When we were supporting v1, we had to take a default branch to push
changes back to as well as jump through hoops with setting up proper
access urls to push to it.

actions/checkout v2 is a lot more featureful. One of those features
is to take a branch (under the input ref) to check out as well as
the token to use to checkout.

Switching to v2 and moving the "default branch" and "token to push/pull
GitHub with" into the checkout step from our scripts is an overall win.

This is a breaking change as I don't want to try to figure out supporting
both v1 and v2.

Closes #32

[ponylang-main]

Hi @SeanTAllen,

The changelog - changed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 34.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.