feat: shallow clone action repositories (#1053)

## Summary

When a workflow references a remote action (e.g. `uses: actions/checkout@v4`) the runner clones that repository during job setup.
Previously this was always a full clone(every branch and the complete history) even though only a single ref is needed.

This PR makes the runner shallow-clone the requested ref by default (`--depth=1`, single branch), falling back to a full clone when a shallow clone fails.

Notes:
- Existing on-disk caches are reused as-is; there is no forced re-clone on upgrade.

## Changes

- A new `runner.action_shallow_clone` option (default `true`) lets operators opt back into full clones.
- `cloneAtDepth`: attempt a shallow clone; fall back to a full clone when shallow clone fails.
- Keep a shallow cache cheap on update: fetch the single requested ref at depth 1 and skip `pull`.

---------

Co-authored-by: bircni <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1053
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-committed-by: Zettat123 <zettat123@gmail.com>

internal/app/run/runner.go

@@ -396,6 +396,12 @@ func (r *Runner) run(ctx context.Context, task *runnerv1.Task, reporter *report.
 		maxLifetime = time.Until(deadline)
 	}
 
+	// shallow clones the requested ref at depth 1, otherwise 0 means a full clone
+	actionCloneDepth := 1
+	if r.cfg.Runner.ActionShallowClone != nil && !*r.cfg.Runner.ActionShallowClone {
+		actionCloneDepth = 0
+	}
+
 	workdirParent := strings.TrimLeft(r.cfg.Container.WorkdirParent, "/")
 	if r.cfg.Container.BindWorkdir {
 		// Append the task ID to isolate concurrent jobs from the same repo.
@@ -418,6 +424,7 @@ func (r *Runner) run(ctx context.Context, task *runnerv1.Task, reporter *report.
 		ActionCacheDir:    filepath.FromSlash(r.cfg.Host.WorkdirParent),
 		AllocatePTY:       r.cfg.Runner.AllocatePTY,
 		ActionOfflineMode: r.cfg.Cache.OfflineMode,
+		ActionCloneDepth:  actionCloneDepth,
 
 		ReuseContainers:      false,
 		ForcePull:            r.cfg.Container.ForcePull,