ci(visual): cache apt packages for heavy Linux DEs

[afonsojramos]

Summary

KDE Plasma, Cinnamon, and Budgie pull hundreds of MB of apt packages on every visual-tray run. Adds awalsh128/cache-apt-pkgs-action@v1.6.0 for those three matrix entries so subsequent runs skip the download phase.

• Split install into base dependencies (xvfb, faketime, libgtk-3-0, etc. — same for every job, fast) + desktop environment (per-matrix).
• New cache_apt: true flag on the KDE/Cinnamon/Budgie matrix entries routes them through the cache action; everything else uses regular apt-get install.
• Action is tag-pinned to v1.6.0 (latest stable, published 2025-10-15) and scoped to version: '1.0' — bump that string to invalidate caches if the heavy DE package set ever changes.

Expected saving: ~30–60s per heavy run on cache hits. Today's KDE/Budgie 17–20min times were mostly GHA queue wait, not install — this won't fix that, but it does trim real wall-clock when runners are responsive.

Test plan

• First run: cache misses on all three, jobs still pass (warm cache populated)
• Second run: cache hits visible in job log, KDE/Cinnamon/Budgie install steps measurably faster