Temporarily override GOV.UK Frontend install

I’ve also had to temporarily adjust `npmInstall()` to install the `govuk-frontend@5.0.0-testing` preview

Otherwise `govuk-frontend@4.7.0` is installed into created prototypes via the registry, despite not having this version locally

bin/utils/index.js

@@ -7,16 +7,23 @@ const packageJsonFormat = { encoding: 'utf8', spaces: 2 }
 
 async function npmInstall (cwd, dependencies) {
   dependencies.push('--save-exact')
+
+  // Temporarily override to GOV.UK Frontend v5 preview (via GitHub)
+  const dependenciesMapped = dependencies
+    .map((packageName) => packageName === 'govuk-frontend'
+      ? 'alphagov/govuk-frontend#6d1a4ef0a'
+      : packageName)
+
   return spawn(
     'npm', [
       'install',
-      ...dependencies
+      ...dependenciesMapped
     ], {
       cwd,
       stderr: 'inherit'
     })
     .catch(e => {
-      console.error('Failed to install dependencies: ', dependencies.join(', '))
+      console.error('Failed to install dependencies: ', dependenciesMapped.join(', '))
       console.error(e)
       process.exit(0)
     })