[CI] Install Clang 10 and use it for the werror build

Summary: The latest version is expected to give the best warnings.

Test Plan: Run the CI build-werror build plan.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6314