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/common/git/git_test.go

@@ -310,11 +310,11 @@ func TestAcquireCloneLock(t *testing.T) {
 	t.Run("same directory serializes", func(t *testing.T) {
 		dir := t.TempDir()
 
-		unlock1 := acquireCloneLock(dir)
+		unlock1 := AcquireCloneLock(dir)
 
 		secondAcquired := make(chan struct{})
 		go func() {
-			unlock := acquireCloneLock(dir)
+			unlock := AcquireCloneLock(dir)
 			close(secondAcquired)
 			unlock()
 		}()
@@ -338,12 +338,12 @@ func TestAcquireCloneLock(t *testing.T) {
 		dirA := t.TempDir()
 		dirB := t.TempDir()
 
-		unlockA := acquireCloneLock(dirA)
+		unlockA := AcquireCloneLock(dirA)
 		defer unlockA()
 
 		done := make(chan struct{})
 		go func() {
-			unlock := acquireCloneLock(dirB)
+			unlock := AcquireCloneLock(dirB)
 			unlock()
 			close(done)
 		}()