[CI] Make build-configurations.sh take the build name as an argument

Summary:
This is a much more natural way to call the script:

./build-configurations.sh build-asan

rather than:

ABC_BUILD_NAME=build-asan ./build-configurations.sh

The environment variable is still accepted in order to maintain
compatibility with existing infra.

Test Plan:

./build-configurations.sh build-clang-10

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6681