Adds built-in actions, native Go actions shipped with the runner and selected with `uses: builtin:<name>`. Jobs need no action download and no Node in their image.
The first one is `builtin:checkout`, a subset of `actions/checkout` with `repository`, `ref`, `token`, `path` and `fetch-depth`. Other inputs fail the step. The runner clones shallowly on its host and copies the tree into the job, while `gitea-runner exec` copies the local working directory for the workflow's own ref.
`actions/checkout` and other remote actions are unchanged. A clone ref of `HEAD` now shallow-clones the remote's default branch, which checkout uses for other repositories.
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Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: silverwind <2021+silverwind@noreply.gitea.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/871
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: bircni <bircni@icloud.com>
