[CI] Run gitian builds through the build configuration

Summary:
This will allow for setting the timeout and make them appear in the
build status panel.

The only change in behavior is that the result directory is now located
under a build directory rather than project root.

Depends on D6948.

Test Plan: Run the gitian-* CI builds.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6949