#4364 use string builder

@@ -35,7 +35,7 @@ public class LibreOfficeRenderer {
             context.put("Linux_X86_64", "$Linux_X86_64");
             context.put("Build", "$Build");
 
-            String output = "";
+            StringBuilder output = new StringBuilder();
             int idx = 0;
 
             while (idx < template.length()) {
@@ -44,12 +44,12 @@ public class LibreOfficeRenderer {
                 String tmplChunk = template.substring(idx, idx + chunkLength);
                 SimpleTemplateEngine engine = new SimpleTemplateEngine();
                 Writable stTemplate = engine.createTemplate(tmplChunk).make(context);
-                output += stTemplate.toString();
+                output.append(stTemplate.toString());
                 idx += chunkLength;
             }
 
             File outputFile = File.createTempFile("bfdcs-", "-tmpl");
-            UtilIO.writeString(outputFile, output);
+            UtilIO.writeString(outputFile, output.toString());
 
             List<String> args = Arrays.asList(new String[]{"libreoffice", "--headless", "--convert-to", "pdf", outputFile.getPath(), "--outdir", "/tmp"});
             ProcessBuilder pb = new ProcessBuilder(args);