[CI] Make sure every build directory is under a common 'build' directory
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.
./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
Differential Revision: https://reviews.bitcoinabc.org/D6719