Avoid picking up the wrong Packages files from previous builds

When running update_test_binaries.sh for two different branches without cleaning up files in between, the Packages indexes get re-used even if they target the wrong release.

This means that if you update the v2022pre binaries, then switch to v2021 and do the update, the tool will re-use the v2022pre indexes and retrieve the v2022pre packages rather than the v2021 ones.

By including the release in the locally cached Packages files we avoid the issue.

Here's an example of a pipeline failing due to this issue: https://gitlab.apertis.org/tests/glib-gio-fs/-/jobs/1184873

Task: APERTIS-8162 (baseline)