[Java] Fix GCC 6 install on Ubuntu 20.04.

Author: vyazelenko

.github/workflows/ci.yml

@@ -112,8 +112,8 @@ jobs:
           key: ${{ runner.os }}-gradle-wrapper-${{ hashFiles('**/gradle/wrapper/gradle-wrapper.properties') }}
       - name: Install compiler
         run: |
+          sudo add-apt-repository "deb http://dk.archive.ubuntu.com/ubuntu/ bionic main universe"
           sudo apt-get install -y g++-${{ matrix.version }} libsparsehash-dev
-
           curl -O -L https://github.com/Kitware/CMake/releases/download/v${CMAKE_VERSION}/cmake-${CMAKE_VERSION}-Linux-x86_64.tar.gz
           tar xf cmake-${CMAKE_VERSION}-Linux-x86_64.tar.gz
           echo "cmake-${CMAKE_VERSION}-Linux-x86_64/bin" >> $GITHUB_PATH