ci: cover C++20 and gcc import in the module install tests

The clang module job now includes C++20, the floor this branch
restores. The gcc job now builds the module itself (CLI11_MODULES=ON)
in addition to the include-based test, at C++20 and C++23.

Assisted-by: ClaudeCode:claude-fable-5

Author: henryiii

.github/workflows/install-tests.yml

@@ -140,19 +140,23 @@ jobs:
         working-directory: build
 
   install-module:
-    name: install module include tests
+    name: install module include tests (gcc, C++${{ matrix.std }})
     runs-on: ubuntu-latest
     container: gcc:15
+    strategy:
+      fail-fast: false
+      matrix:
+        std: ["20", "23"]
     steps:
       - uses: actions/checkout@v4
         with:
           submodules: true
       - uses: ./.github/actions/quick_cmake
         with:
-          cmake-version: "4.2"
+          cmake-version: "4.4"
       - uses: seanmiddleditch/gha-setup-ninja@master
       - name: Configure
-        run: cmake -S . -B build -G Ninja -DCLI11_INSTALL_PACKAGE_TESTS=ON -DCLI11_BUILD_EXAMPLES=OFF -DCLI11_MODULE_TESTS=ON -DCMAKE_CXX_STANDARD=23 -DCMAKE_INSTALL_PREFIX=/home/runner/work/install 
+        run: cmake -S . -B build -G Ninja -DCLI11_INSTALL_PACKAGE_TESTS=ON -DCLI11_BUILD_EXAMPLES=OFF -DCLI11_MODULE_TESTS=ON -DCLI11_MODULES=ON -DCMAKE_CXX_STANDARD=${{ matrix.std }} -DCMAKE_INSTALL_PREFIX=/home/runner/work/install
       - name: Build
         run: cmake --build build -j4
       - name: install
@@ -167,7 +171,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        std: ["23", "26"]
+        std: ["20", "23", "26"]
     steps:
       - name: Set up Clang
         uses: egor-tensin/setup-clang@v2