fix(ci/flatpak): cargo-sources generator needs python3-tomlkit, not toml

flatpak-cargo-generator.py (master) imports `tomlkit` + `aiohttp`; the workflow
installed `python3-toml`, so the "Generate offline cargo sources" step would fail
with ModuleNotFoundError. Install python3-tomlkit instead, and correct the same
note in build-flatpak.sh.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/flatpak.yml

@@ -47,7 +47,8 @@ jobs:
 
       - name: Tooling
         run: |
-          dnf -y install flatpak flatpak-builder git python3 python3-aiohttp python3-toml curl jq
+          # flatpak-cargo-generator.py (master) needs aiohttp + tomlkit (NOT the old `toml`).
+          dnf -y install flatpak flatpak-builder git python3 python3-aiohttp python3-tomlkit curl jq
           # Flathub provides the GNOME runtime/SDK + the rust-stable + ffmpeg-full extensions.
           flatpak remote-add --user --if-not-exists flathub \
             https://dl.flathub.org/repo/flathub.flatpakrepo