[build-configurations] Resurface the more informative error message when ABC_BUILD_NAME is not set

Summary: D3992 ended up masking this error message. Since the custom error message is more informative, I decided to move set -u instead of removing the message entirely. Not having set -u set after simple error messages like this should have negligible effect.

Test Plan:
Run:

./build-configurations.sh

Result before patch:

./build-configurations.sh: line 7: ABC_BUILD_NAME: unbound variable

Result after patch:

+ '[' -z '' ']'
+ echo 'Error: Environment variable ABC_BUILD_NAME must be set'
Error: Environment variable ABC_BUILD_NAME must be set
+ exit 1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4214