Change CI builds to use utf-8 encoding

Summary:
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