Await profile items update in skip

[joshuatf]

Fixes #4467

Fixes the broken skip button on the benefits page.

This was occurring because updateProfileItems now returns a promise so the await that was previously broken began working.

This is part of the problem described in #2959. The current logic is still messy, #2959 can finally be resolved once #4144 is merged in. 🎉

Detailed test instructions:

1. Disable Jetpack and/or WCS.
2. Enable the profiler and visit the last step.
3. Attempt to continue by skipping with "No thanks"
4. Reset the profiler.
5. Attempt to continue by installing plugins with "Yes please"

[jeffstieler]

Code looks good and this tested well.