Cleanup build config

Author: vitaut

.github/workflows/linux.yml

@@ -126,18 +126,16 @@ jobs:
           clang-3.6_3.6.2-3ubuntu2_amd64.deb
       if: ${{ matrix.cxx == 'clang++-3.6' }}
 
-    - name: Add repositories for newer GCC
-      run: |
-        sudo apt-add-repository ppa:ubuntu-toolchain-r/test
-      if: ${{ matrix.cxx == 'g++-11' || matrix.cxx == 'g++-13' }}
-
     - name: Add Ubuntu mirrors
       run: |
         # GitHub Actions caching proxy is at times unreliable
-        # see https://github.com/actions/runner-images/issues/7048
-        printf 'http://azure.archive.ubuntu.com/ubuntu\tpriority:1\n' | sudo tee /etc/apt/mirrors.txt
-        curl http://mirrors.ubuntu.com/mirrors.txt | sudo tee --append /etc/apt/mirrors.txt
-        sudo sed -i 's~http://azure.archive.ubuntu.com/ubuntu/~mirror+file:/etc/apt/mirrors.txt~' /etc/apt/sources.list
+        # see https://github.com/actions/runner-images/issues/7048.
+        printf 'http://azure.archive.ubuntu.com/ubuntu\tpriority:1\n' | \
+          sudo tee /etc/apt/mirrors.txt
+        curl http://mirrors.ubuntu.com/mirrors.txt | \
+          sudo tee --append /etc/apt/mirrors.txt
+        sudo sed -i 's~http://azure.archive.ubuntu.com/ubuntu/~mirror+file:/etc/apt/mirrors.txt~' \
+          /etc/apt/sources.list
 
     - name: Create build environment
       run: |