refactor(repo): Use byte buffer to build index file

The string returned by the string builder was cast to a byte slice
anyways. Remove this indirection. Also, write directly to the buffer
instead of using fmt.Fprintln(...).

Signed-off-by: Tom Wieczorek <twieczorek@mirantis.com>

pkg/repo/v1/chartrepo.go

@@ -104,14 +104,15 @@ func (r *ChartRepository) DownloadIndexFile() (string, error) {
 	}
 
 	// Create the chart list file in the cache directory
-	var charts strings.Builder
+	var charts bytes.Buffer
 	for name := range indexFile.Entries {
-		fmt.Fprintln(&charts, name)
+		charts.WriteString(name)
+		charts.WriteByte('\n') // Terminate each entry with a newline
 	}
 	chartsFile := filepath.Join(r.CachePath, helmpath.CacheChartsFile(r.Config.Name))
 	os.MkdirAll(filepath.Dir(chartsFile), 0o755)
 
-	fileutil.AtomicWriteFile(chartsFile, bytes.NewReader([]byte(charts.String())), 0o644)
+	fileutil.AtomicWriteFile(chartsFile, &charts, 0o644)
 
 	// Create the index file in the cache directory
 	fname := filepath.Join(r.CachePath, helmpath.CacheIndexFile(r.Config.Name))