Normalize path outputs emitted by the artifact server download endpoint (#1898)

Co-authored-by: Casey Lee <cplee@nektos.com>

pkg/artifacts/server.go

@@ -223,9 +223,13 @@ func downloads(router *httprouter.Router, baseDir string, fsys fs.FS) {
 
 				// if it was upload as gzip
 				rel = strings.TrimSuffix(rel, gzipExtension)
+				path := filepath.Join(itemPath, rel)
+
+				rel = filepath.ToSlash(rel)
+				path = filepath.ToSlash(path)
 
 				files = append(files, ContainerItem{
-					Path:            filepath.Join(itemPath, rel),
+					Path:            path,
 					ItemType:        "file",
 					ContentLocation: fmt.Sprintf("http://%s/artifact/%s/%s/%s", req.Host, container, itemPath, rel),
 				})