Update build-linux.yml workflow

We don't need to package the result for flathub anymore as this happens on
https://github.com/flathub/de.wger.flutter on flathub's infrastructure. We just
upload the linux builds to the github releases.

.github/workflows/build-linux.yml

@@ -6,9 +6,20 @@ on:
         required: true
         type: string
 jobs:
+
+  # Note that we currently only build for x64, as arm64 is not supported by
+  # subosito/flutter-action@v2 yet and we don't want to install flutter manually
+  # just for this workflow: https://github.com/subosito/flutter-action/issues/345
   build_linux:
-    name: Flathub
-    runs-on: ubuntu-latest
+    name: Build application - ${{ matrix.platform }}
+    runs-on: ${{ matrix.runner }}
+    strategy:
+      matrix:
+        include:
+          - platform: x64
+            runner: ubuntu-latest
+        # - platform: arm64
+        #   runner: ubuntu-24.04-arm
     steps:
       - name: Checkout application
         uses: actions/checkout@v4
@@ -22,17 +33,13 @@ jobs:
       # https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2404-Readme.md
       - name: Build application for linux
         run: |
-          # ninja-build 
           sudo apt update
           sudo apt install -y pkg-config libgtk-3-dev liblzma-dev libstdc++-12-dev --no-install-recommends
           flutter build linux --release
-          cd flatpak/scripts
-          dart pub get
-          dart flatpak_packager.dart --meta ../flatpak_meta.json --addTodaysVersion ${{inputs.ref}}
-
+          tar -zcvf linux-${{ matrix.platform }}.tar.gz build/linux/${{ matrix.platform }}/release/bundle
+          cp build/linux/${{ matrix.platform }}/release/bundle/wger build/linux/${{ matrix.platform }}/release/bundle/wger-linux-${{ matrix.platform }}
       - uses: actions/upload-artifact@v4
         with:
           name: builds-linux
           path: |
-            flatpak/scripts/flatpak_generator_exports/wger-linux-x86_64.tar.gz
-            flatpak/scripts/flatpak_generator_exports/wger-linux-x86_64.sha256
+            linux-${{ matrix.platform }}.tar.gz