child_process: refactor normalizeSpawnArguments()

Code is not in hot path. Refactor ternary to be more readable if/else
block that mirrors analogous code elsewhere in the function. Change
string concatenation to template literal.

PR-URL: #14149
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

lib/child_process.js

@@ -440,9 +440,11 @@ function normalizeSpawnArguments(file, args, options) {
     const command = [file].concat(args).join(' ');
 
     if (process.platform === 'win32') {
-      file = typeof options.shell === 'string' ? options.shell :
-              process.env.comspec || 'cmd.exe';
-      args = ['/d', '/s', '/c', '"' + command + '"'];
+      if (typeof options.shell === 'string')
+        file = options.shell;
+      else
+        file = process.env.comspec || 'cmd.exe';
+      args = ['/d', '/s', '/c', `"${command}"`];
       options.windowsVerbatimArguments = true;
     } else {
       if (typeof options.shell === 'string')