[CI] Make sure every build directory is under a common 'build' directory

Summary:
D6551 introduced naming the build directory based on the build name.
Although currently manageable today, it's implicitly adding a restriction on
build names such that they may not collide with any file names at the project root.
As iteration speed increases, this silent restriction may become a hinderance,
either as files in the repository are refactored/renamed or as the build config
naming scheme changes over time.

We can fix this by using a common build directory at the top-level, but still
use the naming scheme introduced in D6551.

Depends on D6711 and D6718

Test Plan:

./build-configurations.py build-without-wallet
ls <toplevel>/build/build-without-wallet        # verify this exists

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6719