CI: install g++-8 for ubuntu 18.04

.github/workflows/build.yml

@@ -99,6 +99,11 @@ jobs:
       if: startswith(matrix.config.os, 'ubuntu')
       run: sudo apt install qt5-default libqt5svg5-dev
     
+    # Ubuntu 18.04 only:
+    - name: Install g++-8
+      if: matrix.config.os == 'ubuntu-18.04'
+      run: sudo apt install g++-8
+
     # All:
     - name: Create Build Environment
       run: cmake -E make_directory ${{github.workspace}}/client/build
@@ -242,6 +247,11 @@ jobs:
       if: startswith(matrix.config.os, 'ubuntu')
       run: sudo apt install liblua5.3-dev lcov libarchive-dev
 
+    # Ubuntu 18.04 only:
+    - name: Install g++-8
+      if: matrix.config.os == 'ubuntu-18.04'
+      run: sudo apt install g++-8
+
     # All:
     - name: Create Build Environment
       # Some projects don't allow in-source building, so create a separate build directory