[CI] Make the build configuration a class

Summary:
This is purely a refactor to make the main contain only the build logic,
and limit the amount of scoped variables.
The build will also be extracted as a class in a later diff.

Depends on D6748.

Test Plan: Run any CI build.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6749