fix: serialize action-cache reads to prevent worktree race (#938)

`NewGitCloneExecutor` holds a per-directory mutex while it `git checkout --force`s a remote action into the shared `<ActionCacheDir>/<UsesHash>`, but four read sites ran unlocked:

- `maybeCopyToActionDir`'s tar walk via `JobContainer.CopyDir`
- `prepareActionExecutor`'s `readAction` parse of `action.yml`
- `newReusableWorkflowExecutor`'s `model.NewWorkflowPlanner` after `cloneRemoteReusableWorkflow` released its lock
- `execAsDocker` when `ActionCache == nil`: `docker build` walks `contextDir` for the daemon-side build context

When two matrix jobs share a `uses:`, a read interleaved with a peer's checkout produces partial state — observed as `Cannot find module .../dist/index.js` and `setup-uv` failing on a half-written `action.yml`.

Exports `acquireCloneLock` as `AcquireCloneLock` and takes it at all four sites. `container.ImageExistsLocally` / `NewDockerBuildExecutor` and `model.NewWorkflowPlanner` are indirected through package-level vars so the docker-action build path and the reusable-workflow read site are testable without a real daemon, mirroring `ContainerNewContainer`. Three regression tests cover the higher-risk sites (`maybeCopyToActionDir`, `execAsDocker`, `newReusableWorkflowExecutor`); each fails if its `AcquireCloneLock` is removed.

Subsumed by https://gitea.com/gitea/runner/pulls/814 once that lands. Related: https://gitea.com/gitea/runner/pulls/930

---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: Nicolas <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/938
Reviewed-by: Nicolas <bircni@icloud.com>
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

act/runner/step_action_remote.go

@@ -145,6 +145,7 @@ func (sar *stepActionRemote) prepareActionExecutor() common.Executor {
 		return common.NewPipelineExecutor(
 			ntErr,
 			func(ctx context.Context) error {
+				defer git.AcquireCloneLock(actionDir)()
 				actionModel, err := sar.readAction(ctx, sar.Step, actionDir, sar.remoteAction.Path, remoteReader(ctx), os.WriteFile)
 				sar.action = actionModel
 				return err