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]>

act/common/git/git.go

@@ -438,10 +438,11 @@ func cloneAtDepth(ctx context.Context, input NewGitCloneExecutorInput, opts git.
 				return nil, err
 			}
 		} else {
-			for _, refName := range []plumbing.ReferenceName{
-				plumbing.NewBranchReferenceName(input.Ref),
-				plumbing.NewTagReferenceName(input.Ref),
-			} {
+			refNames := []plumbing.ReferenceName{plumbing.NewBranchReferenceName(input.Ref), plumbing.NewTagReferenceName(input.Ref)}
+			if input.Ref == plumbing.HEAD.String() || strings.HasPrefix(input.Ref, "refs/") {
+				refNames = []plumbing.ReferenceName{plumbing.ReferenceName(input.Ref)} // HEAD is the remote's default branch
+			}
+			for _, refName := range refNames {
 				shallowOpts := opts
 				shallowOpts.Depth = input.Depth
 				shallowOpts.SingleBranch = true