Migrate Onion Launchpad CI to Debian trixie

Description

Seems like Lektor CI was recently updated to allow for building on Debian bookwormtrixie:

• switch to containers.tpo images (e69cf026).
• restore venv for debian imgaes compatibility (6a37045b).
• lektor: install curl for dart sass installation (da4dd6a0).
• move curl into build script where its needed (7e1ab223).
• specify image for check-l10n-updates job (c34f364e).

This ticket supersedes !76 (merged).

Tasks

• Fix issue with pip and setuptools (tpo/web/lego!99 (merged)).
• Update Onion Launchpad's CI templates for https://gitlab.torproject.org to use a Debian bookworm image.
• Update Onion Launchpad's CI templates for https://gitlab.com to use a Debian bookworm image.
• Update Onion Launchpad's CI templates for https://github.com to use a Debian bookworm image.

Time estimation

• Complexity: small (1 day)
• Uncertainty: low (x1.1)
• Reference (adapted)