Build an experimental chronik release

Summary:
This adds a chronik-experimental variant to the gitian 64 bit linux
build. This is intended to be replaced by GUIX at some point but will
help deploying chronik in the meantime.

Test Plan: Run the linux gitian build.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13952