[CI] Split build-configurations.sh into individual scripts

Summary:
This is another step toward configuring the builds individually from the
VCS. There is no change in behavior.

Depends on D6701.

Test Plan: Run any CI build plan.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6703