[CI] Install git-filter-repo from the backport debian apt repository

Summary: The package doesn't exist in the main bullseye repository and is only available through the backport repo.

Test Plan: Call the install script from a fresh bullseye-slim docker image

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14218