[CI] Always move the Gitian install and build log

Summary:
Due to set -e, if the gitian build fails the log is not moved to the
output directory. This diff traps the error and move the log.

Test Plan: Run the gitian build with a failure.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4750