use a modern gcc on linux

.github/workflows/build-test.yml

@@ -17,6 +17,13 @@ jobs:
       - uses: actions/checkout@v4
         with:
           submodules: true
+
+      - name: switch to gcc-12 on linux
+        if: matrix.os == 'ubuntu-latest'
+        run: |
+          sudo apt install gcc-12 g++-112
+          sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-12 100 --slave /usr/bin/g++ g++ /usr/bin/g++-12 --slave /usr/bin/gcov gcov /usr/bin/gcov-12
+          sudo update-alternatives --set gcc /usr/bin/gcc-12
       
       - uses: lukka/get-cmake@latest