fix(ci): give the builder image what linuxdeploy needs for the AppImage

The v0.10.0 release build failed in Build Linux after 16 minutes:

  failed to bundle project: xdg-open binary not found
  /usr/bin/xdg-open: No such file or directory

linuxdeploy embeds xdg-open into the AppImage and aborts the whole bundle
when it is absent. deb and rpm had already bundled fine; only AppImage
was affected.

This is the one failure tonight that building locally could not have
caught, and the reason is worth writing down: a developer machine is a
desktop and always has xdg-utils, so the AppImage builds there and fails
on a minimal server image. The asymmetry is the bug. Every other release
defect this evening was found by building locally first; this one needed
the runner.

xdg-utils, desktop-file-utils and zsync are added together rather than
one at a time. Each round trip costs an image rebuild plus a failed
release build, and those three are what linuxdeploy commonly reaches for
(xdg-open, desktop-file-validate, and zsync for delta updates).

Workflows move to jellytau-builder:2026.08.1, built and pushed with all
three verified present inside it before this commit.

ci-operations.md gains two things learned here: that an apt addition
invalidates the layer above the cargo-install steps, so it is a ~20 minute
rebuild rather than the ~2 minutes the trailing layer normally gives; and
that Tauri's AppImage bundler downloads linuxdeploy, AppRun and two plugin
scripts from GitHub during the build, so an AppImage build depends on
GitHub being reachable from the runner.

.gitea/workflows/publish-docs.yml

@@ -21,7 +21,7 @@ jobs:
     name: Build & publish docs to gitea-pages
     runs-on: linux/amd64
     container:
-      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08
+      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08.1
 
     steps:
       - name: Checkout code