Make the build script mac friendly.
Make the build script mac friendly.

-R at the end will throw an error like (failing the npm install):

```
Error: Command failed: /bin/sh -c cp /myproject/node_modules/hmpo-frontend-toolkit/node_modules/govuk_frontend_toolkit/images/* ./assets/images/icons/ -R
usage: cp [-R [-H | -L | -P]] [-fi | -n] [-apvX] source_file target_file
       cp [-R [-H | -L | -P]] [-fi | -n] [-apvX] source_file ... target_directory

    at ChildProcess.exithandler (child_process.js:751:12)
    at ChildProcess.emit (events.js:110:17)
    at maybeClose (child_process.js:1015:16)
    at Socket.<anonymous> (child_process.js:1183:11)
    at Socket.emit (events.js:107:17)
    at Pipe.close (net.js:485:12)
```

UKHomeOffice/passports-frontend-toolkit