Move from checkout@v1 to checkout@v2

[juliasilge]

Describe the bug
When jobs are re-run, a race condition (? or similar) in checkout@v1 causes jobs to error with the message fatal: reference is not a tree. 🚫 🌳

The Actions team has fixed this problem in actions/checkout@v2 (discussion in actions/checkout#23) so it would be great to move up the checkout version in check-full.yaml and similar.

To Reproduce
Here is a PR that is experiencing this behavior. Every time I click "Re-run jobs", it fails at actions/checkout@v1.

Expected behavior
When jobs are re-run, the checkout succeeds.

[github-actions]

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