[gitian-build] Refactor to calculate output directories in fewer places

Summary: This makes changes less error prone.

Test Plan:

./gitian-build.py -n -c -b gitianuser v0.20.3

Verify gitian build binaries still output as expected.

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4231