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.

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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]>

README.md

@@ -306,6 +306,10 @@ Dockerfile actions are built with these variables as build arguments, so their `
 
 A password in a proxy URL is hidden in job logs. Any step can still read it, because the step is given the proxy URL in its environment.
 
+#### Built-in actions (`uses: builtin:<name>`)
+
+Use `uses: builtin:checkout` to check out a repository without downloading `actions/checkout` or needing Node in the job image, see **[docs/builtin-actions.md](docs/builtin-actions.md)**.
+
 #### Caching (`actions/cache`)
 
 Each runner starts its own cache server, so runners do not share cached entries. When the runner itself runs in Docker, set `cache.host` to an address job containers can reach and `cache.port` to a fixed published port, or put jobs on a shared `container.network`.