Build using GCC 9 in CI

Checking that we don't regress this later.
Ubuntu doesn't package GCC 8, and we don't support it either,
so that is a good compromise for now.

.github/workflows/testing.yml

@@ -40,6 +40,9 @@ jobs:
         exclude: # Don't use `g++` on macOS; it's just an alias to `clang++`.
           - { os: macos-15-intel, cxx: g++ }
           - { os: macos-26, cxx: g++ }
+        include: # Use `g++-9`, the earliest GCC version we support, on the earliest Ubuntu runner.
+          - { os: ubuntu-22.04, cxx: g++-9, buildsys: make }
+          - { os: ubuntu-22.04, cxx: g++-9, buildsys: cmake }
       fail-fast: false
     runs-on: ${{ matrix.os }}
     steps:
@@ -47,7 +50,7 @@ jobs:
         uses: actions/checkout@v6
       - name: Install deps
         run: |
-          .github/scripts/install_deps.sh ${{ matrix.os }}
+          .github/scripts/install_deps.sh ${{ matrix.os }} ${{ matrix.cxx }}
       - name: Build & install using Make
         if: matrix.buildsys == 'make'
         run: |