Fix install command with older versions of npm

Older versions of Node come with npm@6, which doesn't have the npm exec
command (and may or may not have npx). Instead of relying on that
tool just call the binary directly. This also means we can ensure we're
using the same version of node as the caller.

bin/cli

@@ -98,9 +98,10 @@ function isSafeToCreateProjectIn (root, name) {
       const packageSpec = argv[4] || 'govuk-prototype-kit'
 
       await fs.writeFile(path.join(installDirectory, 'package.json'), '{}' + os.EOL, 'utf8')
-      execSync(`npm install ${packageSpec}`, { stdio: 'inherit' })
+      console.log(`Installing ${packageSpec}, this may take a few minutes...`)
+      execSync(`npm install ${packageSpec}`, { stdio: 'ignore' })
       execSync(
-        `npm exec -- govuk-prototype-kit install -- ${installDirectory}`,
+        `${process.execPath} node_modules/.bin/govuk-prototype-kit install -- ${installDirectory}`,
         {
           env: { LANG: process.env.LANG, PATH: process.env.PATH }, // clearing the env of any npm_ stuff is important,
                                                                    // otherwise npx won't be able to use the package we just installed