lib: replace string concatenation with template

PR-URL: #16920
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

lib/child_process.js

@@ -266,7 +266,7 @@ exports.execFile = function(file /*, args, options, callback*/) {
     }
 
     if (args.length !== 0)
-      cmd += ' ' + args.join(' ');
+      cmd += ` ${args.join(' ')}`;
 
     if (!ex) {
       ex = new Error('Command failed: ' + cmd + '\n' + stderr);
@@ -475,7 +475,7 @@ function normalizeSpawnArguments(file, args, options) {
   var envPairs = [];
 
   for (var key in env) {
-    envPairs.push(key + '=' + env[key]);
+    envPairs.push(`${key}=${env[key]}`);
   }
 
   _convertCustomFds(options);
@@ -570,7 +570,7 @@ function checkExecSyncError(ret, args, cmd) {
     var msg = 'Command failed: ';
     msg += cmd || args.join(' ');
     if (ret.stderr && ret.stderr.length > 0)
-      msg += '\n' + ret.stderr.toString();
+      msg += `\n${ret.stderr.toString()}`;
     err = new Error(msg);
   }
   if (err) {