Set default compiler to gcc-9 on ubuntu-latest (#224)

* Set default compiler to gcc-9 on ubuntu-latest

* Update linux.yml

* Update linux.yml

.github/workflows/linux.yml

@@ -23,6 +23,12 @@ jobs:
         run: |
           # TODO we will remove it later
           sudo apt-get install -y libgl1-mesa-dev libglu1-mesa-dev
+          sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-9 60
+          sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-9 60
+          sudo update-alternatives --install /usr/bin/cpp cpp /usr/bin/cpp-9 60
+          sudo update-alternatives --set g++ /usr/bin/g++-9
+          sudo update-alternatives --set gcc /usr/bin/gcc-9
+          sudo update-alternatives --set cpp /usr/bin/cpp-9
 
       - name: Tests
         run: |