Force rescent compiler on linux

.github/workflows/build.yml

@@ -56,7 +56,8 @@ jobs:
         run: |
           sudo add-apt-repository ppa:ubuntu-toolchain-r/test
           sudo apt-get update
-          sudo apt install libgl1-mesa-dev libxkbcommon-x11-0 gcc-9
+          sudo apt-get install libgl1-mesa-dev libxkbcommon-x11-0 g++-9
+          echo ::set-env name=QMAKE_FLAGS::QMAKE_CXX=g++-9 QMAKE_CC=gcc-9
 
       - name: Get Qt
         run: python ./share/ci/get_qt.py

share/ci/build.py

@@ -19,6 +19,6 @@ c.run('lupdate "{}"'.format(pro_file))
 c.run('lrelease "{}"'.format(pro_file))
 
 c.set_make_threaded()
-c.run('qmake "{}"'.format(pro_file))
+c.run('qmake {} "{}"'.format(os.environ.get('QMAKE_FLAGS',''), pro_file))
 make_cmd = c.get_make_cmd()
 c.run(make_cmd)