fix artifact merging chunks path with correct slash on Windows (#26400)

From Discord
https://discord.com/channels/322538954119184384/1069795723178160168/1136719889684500480

Artifact chunks merging is break on Windows.

```
Gitea Log:
2023/08/03 20:51:15 ...actions/artifacts.go:271:comfirmUploadArtifact() [E] Error merge chunks: parse content range error: input does not match format
```

Artifact uses wrong slash to parse saved chunks path.

routers/api/actions/artifacts_chunks.go

@@ -8,6 +8,7 @@ import (
 	"encoding/base64"
 	"fmt"
 	"io"
+	"path/filepath"
 	"sort"
 	"time"
 
@@ -67,7 +68,7 @@ func listChunksByRunID(st storage.ObjectStorage, runID int64) (map[int64][]*chun
 	var chunks []*chunkFileItem
 	if err := st.IterateObjects(storageDir, func(path string, obj storage.Object) error {
 		item := chunkFileItem{Path: path}
-		if _, err := fmt.Sscanf(path, storageDir+"/%d-%d-%d.chunk", &item.ArtifactID, &item.Start, &item.End); err != nil {
+		if _, err := fmt.Sscanf(path, filepath.Join(storageDir, "%d-%d-%d.chunk"), &item.ArtifactID, &item.Start, &item.End); err != nil {
 			return fmt.Errorf("parse content range error: %v", err)
 		}
 		chunks = append(chunks, &item)