[CI] Set default git user and email during setup

Summary:
Git has a hard requirement that user name/email is configured for
any operation that creates a commit. Since this is practically always configured
during local testing, it's easily missed as a requirement for these types of
operations to work as expected in CI.

To ensure these operations just work, we can set a sane default. Any config
differing from the norm can be reconfigured after setup.

Test Plan:
Run the setup script in a fresh Debian container and verify:

git config user.name
git config user.email

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7773