Update live-bootstrap
This MR updates live-bootstrap to commit 88ea21dfc94ac6ed63f64dccf403e94d1d901665, which is the last upstream commit that doesn't require support for fetching source packages usig git.
Updating further would require upstream changes to expose the new git:// sources in the source_manifest.py script, and respective handling in the live_bootstrap_manifest plugin.
It wasn't feasible to break down the main commit to multiple smaller commits due to the fact that elements and plugins in this repository are meant to follow upstream instructions and bootstrap behavior, and both of those changed in multiple ways as part of the update.
A few additional changes in separate commits are included in this MR to avoid excessive load on CI due to repeated builds.
See commit messages for more information.
/cc @nanonyme