feat(runner): add built-in actions framework with checkout (#871)

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.

---------

Co-authored-by: silverwind <[email protected]>
Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/871
Reviewed-by: silverwind <[email protected]>
Co-authored-by: bircni <[email protected]>

docs/builtin-actions.md

@@ -0,0 +1,21 @@
+# Built-in actions
+
+Built-in actions ship with the runner, so a job needs no action download and no Node in its image.
+
+```yaml
+- uses: builtin:checkout
+```
+
+## checkout
+
+Works like `actions/checkout` with these inputs. Any other input fails the step.
+
+| Input | Default |
+| --- | --- |
+| `repository` | `github.repository` |
+| `ref` | the triggering commit, or the default branch of another repository |
+| `token` | `github.token` |
+| `path` | workspace root |
+| `fetch-depth` | `1`, `0` for full history |
+
+With `gitea-runner exec`, checking out the workflow's own repository copies your local working directory, unless `--no-skip-checkout` is set.