Set buildpack for paas-uaa-assets app.
Set buildpack for paas-uaa-assets app.

This prevents it doing the buildpack detection process, which slows the
deployment down as it has to download all the buildpacks to check.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf