Github actions: Install Qt 5.15

... We use a specific Github Action for this, since the current Qt
version shipping with Ubuntu is too old.

.github/workflows/qmake.yml

@@ -13,8 +13,25 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: install
+    - name: install Qt
-      run: sudo apt-get update -qq && sudo apt-get install qtconnectivity5-dev qtbase5-dev qttools5-dev qtmultimedia5-dev libqt5opengl5-dev qtpositioning5-dev qtdeclarative5-dev qtscript5-dev libqt5sensors5-dev libqt5serialport5-dev libqt5svg5-dev libqt5webkit5-dev libqt5websockets5-dev libqt5x11extras5-dev libqt5xmlpatterns5-dev qml-module-qtquick-privatewidgets qtbase5-private-dev qtdeclarative5-private-dev qttools5-private-dev qtwebengine5-private-dev qt5-default -y
+      uses: jurplel/install-qt-action@v2
+      with:
+        version: '5.15.2'
+        host: 'linux'
+        target: 'desktop'
+        install-deps: 'true'
+        modules: 'qtwebengine'
+        cached: 'false'
+        setup-python: 'true'
+        tools: ''
+        set-env: 'true'
+        tools-only: 'false'
+
+    - name: Ubuntu and Qt version
+      run: |
+        cat /etc/issue
+        echo number of processors: $(nproc)
+        qmake -v
     - name: qmake
       run: qmake qskinny.pro
     - name: make