Pull the new upstream Flatdeb changes and use additional_base_packages

In Soldier we install steamrt-customizations to use the apt pinning preferences just after the debootstrap run.

• Scout test build https://jenkins.internal.steamos.cloud/job/playground/job/scout/job/build/280/
• Soldier test build https://jenkins.internal.steamos.cloud/job/playground/job/soldier/job/build/97/

/cc @smcv