Make compiler variable const instead of let (#6621)

compiler is not being reassigned, so we can use a const here

Author: Primajin

packages/react-scripts/scripts/build.js

@@ -143,7 +143,7 @@ checkBrowsers(paths.appPath, isInteractive)
 function build(previousFileSizes) {
   console.log('Creating an optimized production build...');
 
-  let compiler = webpack(config);
+  const compiler = webpack(config);
   return new Promise((resolve, reject) => {
     compiler.run((err, stats) => {
       let messages;