Use HTTPS for LLVM repository

Summary:
LLVM has migrated their HTTP URLs to 301 (permanent redirect) to HTTPS
for their APT repositories, so we can avoid a redirect by using https directly.

Test Plan: Build the CI base image locally

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6039