Build Flutter app in container.

The Flutter app also becomes dependent on the glibc version available on
the machine, so we need to build it on an older OS.

.github/workflows/linux.yml

@@ -3,57 +3,28 @@ name: Linux
 on: [push, pull_request]
 
 jobs:
-  helper:
-    name: Build helper
+  build:
+
     runs-on: ubuntu-latest
     env:
       PYVER: 3.11
+      FLUTTER: '3.3.9'
     container:
       image: ubuntu:18.04
       env:
         DEBIAN_FRONTEND: noninteractive
 
     steps:
-    - uses: actions/checkout@v3
-
-    - name: Cache helper
-      id: cache-helper
-      uses: actions/cache@v3
-      with:
-        path: helper.tar.gz
-        key: ${{ runner.os }}-py${{ env.PYVER }}-${{ hashFiles('helper/**') }}
-
     - name: Install dependencies
-      if: steps.cache-helper.outputs.cache-hit != 'true'
       run: |
         apt-get update
-        apt-get install -qq swig libpcsclite-dev software-properties-common build-essential cmake git
+        apt-get install -qq software-properties-common
+        add-apt-repository -y ppa:git-core/ppa
         add-apt-repository -y ppa:deadsnakes/ppa
-        apt-get install -qq python$PYVER-dev python$PYVER-venv
-        python$PYVER -m ensurepip --user
-        python$PYVER -m pip install -U pip
-        python$PYVER -m pip install poetry
+        apt-get install -qq git python$PYVER-dev python$PYVER-venv
+        git config --global --add safe.directory "$GITHUB_WORKSPACE"
+        ln -s `which python$PYVER` /usr/local/bin/python
 
-    - name: Build the Helper
-      if: steps.cache-helper.outputs.cache-hit != 'true'
-      run: ./build-helper.sh
-
-    - name: Create archive
-      if: steps.cache-helper.outputs.cache-hit != 'true'
-      run: tar -czf helper.tar.gz build/linux/helper assets/licenses/helper.json
-
-    - name: Upload artifact
-      uses: actions/upload-artifact@v3
-      with:
-        name: helper
-        path: helper.tar.gz
-
-  build:
-    name: Build app
-    runs-on: ubuntu-latest
-    needs: helper
-
-    steps:
     - uses: actions/checkout@v3
 
     - name: Check app versions
@@ -61,26 +32,41 @@ jobs:
         python set-version.py
         git diff --exit-code
 
-    - name: Download helper
-      uses: actions/download-artifact@v3
+    - name: Cache helper
+      id: cache-helper
+      uses: actions/cache@v3
       with:
-        name: helper
+        path: |
+          build/linux/helper
+          assets/licenses/helper.json
+        key: ${{ runner.os }}-py${{ env.PYVER }}-${{ hashFiles('helper/**') }}
 
-    - name: Unpack helper
+    - name: Install helper dependencies
+      if: steps.cache-helper.outputs.cache-hit != 'true'
       run: |
-        tar -xf helper.tar.gz
+        apt-get install -qq swig libpcsclite-dev build-essential cmake
+        python -m ensurepip --user
+        python -m pip install -U pip
+        python -m pip install poetry
 
-    - name: Install dependencies
+    - name: Build the Helper
+      if: steps.cache-helper.outputs.cache-hit != 'true'
+      run: ./build-helper.sh
+
+    - name: Install Flutter dependencies
       run: |
-        sudo apt-get update
-        sudo apt-get install -qq clang cmake ninja-build pkg-config libgtk-3-dev
+        apt-get install -qq clang cmake ninja-build pkg-config libgtk-3-dev jq
 
     - uses: subosito/flutter-action@v2
       with:
         channel: 'stable'
-        flutter-version: '3.3.9'
-    - run: flutter config --enable-linux-desktop
-    - run: flutter --version
+        flutter-version: ${{ env.FLUTTER }}
+
+    - name: Configure Flutter
+      run: |
+        git config --global --add safe.directory /opt/hostedtoolcache/flutter/stable-$FLUTTER-x64
+        flutter config --enable-linux-desktop
+        flutter --version
 
     - name: Run tests
       run: |