Stop installing govuk-prototype-kit globally in tests

We don't want to accidentally end up with development code installed in
system Node installation after running tests.

__tests__/util/index.js

@@ -81,14 +81,9 @@ async function mkPrototype (prototypePath, {
     // Create test starter project folder
     await fs.mkdirp(prototypePath)
 
-    // Generate starter project and start
+    // Generate starter project
     await exec(
-      'npm i -g'
-    )
-
-    // Generate starter project and start
-    await exec(
-      'govuk-prototype-kit create --version local',
+      `${process.execPath} ${repoDir}/bin/cli create --version local`,
       { cwd: prototypePath, env: { ...process.env, env: 'test' } }
     )