refactor!: remove the builtin actions (#1407)

Removes `builtin:checkout`, `builtin:upload-artifact` and `builtin:download-artifact`, along with the container helpers only they used.

Builtin actions are tied to the runner version, so users cannot pin them, and they diverge from the shareable actions ecosystem that tools like zizmor build on. Workflows should use `actions/checkout`, `actions/upload-artifact` and `actions/download-artifact`, or lightweight third-party actions such as https://github.com/taiki-e/checkout-action.

**Breaking:** steps with `uses: builtin:checkout`, released in v4.0.0, now fail and need to switch to one of the actions above. The artifact actions were never released.

Docs: https://gitea.com/gitea/docs/pulls/643
Fixes: https://gitea.com/gitea/runner/issues/1405
Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1407
Reviewed-by: silverwind <[email protected]>
Co-authored-by: bircni <[email protected]>

DEVELOPMENT.md

@@ -51,11 +51,6 @@ fields.
 - Tests read what was logged with `logtest.NewLogger()`, or `logtest.SetDefault(t)` for the
   package functions.
 
-## Built-in actions
-
-Each lives in `internal/pkg/action/<name>` and is registered in `builtinActions` in
-`act/runner/step_action_builtin.go`.
-
 ## End-to-end compatibility tests
 
 `make test-e2e` runs the runner against `E2E_GITEA_IMAGE`. It defaults to the nightly image.