[CI] Add gettext-base to the base image

Summary:
This package provides the envsubst command that is used in some
scripts.

Test Plan:
If the CI builds for this diff are green, then the installation
succeeded.

Reviewers: #bitcoin_abc, Fabien, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7451