Change CI builds to use utf-8 encoding
With the codeset unset, the builds are not guaranteed to use UTF-8, so now we set it explicitly.
This fixes the build issue discovered in D4174.
Depends on D4183
Test Plan: ABC_BUILD_NAME=build-default ./build-configurations.sh
Reviewers: #bitcoin_abc, deadalnix, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D4184