ci: fix flatpak build on publish (#404)

Author: kantord

.github/workflows/on-release.yml

@@ -64,6 +64,15 @@ jobs:
           apple-issuer-id: ${{ secrets.APPLE_ISSUER_ID }}
           apple-key-id: ${{ secrets.APPLE_KEY_ID }}
 
+      - name: Install Flatpak tool-chain (Linux only)
+        if: runner.os == 'Linux'
+        run: |
+          sudo apt-get update
+          # flatpak itself isn’t strictly required to *build*, but it’s good to have.
+          sudo apt-get install -y --no-install-recommends \
+               flatpak flatpak-builder elfutils
+          flatpak remote-add --if-not-exists --user flathub https://dl.flathub.org/repo/flathub.flatpakrepo
+
       - name: Build and Publish to GitHub Release
         run: pnpm run publish
         env: