Add C++20 windows test but only run it on main ci, not on pr ci

Author: Wumpf

.github/workflows/cpp_matrix_full.json

@@ -5,25 +5,36 @@
             "runs_on": "ubuntu-latest-16-cores",
             "cache_key": "build-linux",
             "extra_env_vars": "RERUN_USE_ASAN=1 RERUN_SET_CXX_VERSION=17 LSAN_OPTIONS=suppressions=.github/workflows/lsan_suppressions.supp",
-            "additional_commands": "pixi run -e cpp cpp-docs"
+            "additional_commands": "pixi run -e cpp cpp-docs",
+            "pr_ci": true
         },
         {
             "name": "Linux x64, C++20",
             "runs_on": "ubuntu-latest-16-cores",
             "cache_key": "build-linux",
-            "extra_env_vars": "RERUN_USE_ASAN=1 RERUN_SET_CXX_VERSION=20 LSAN_OPTIONS=suppressions=.github/workflows/lsan_suppressions.supp"
+            "extra_env_vars": "RERUN_USE_ASAN=1 RERUN_SET_CXX_VERSION=20 LSAN_OPTIONS=suppressions=.github/workflows/lsan_suppressions.supp",
+            "pr_ci": false
         },
         {
-            "name": "Windows x64",
+            "name": "Windows x64, C++17",
             "runs_on": "windows-latest-8-cores",
             "cache_key": "build-windows",
-            "extra_env_vars": ""
+            "extra_env_vars": "RERUN_SET_CXX_VERSION=17",
+            "pr_ci": true
+        },
+        {
+            "name": "Windows x64, C++20",
+            "runs_on": "windows-latest-8-cores",
+            "cache_key": "build-windows",
+            "extra_env_vars": "RERUN_SET_CXX_VERSION=20",
+            "pr_ci": false
         },
         {
             "name": "Mac aarch64",
             "runs_on": "macos-15-large",
             "cache_key": "build-macos-arm64",
-            "extra_env_vars": "RERUN_USE_ASAN=1 LSAN_OPTIONS=suppressions=.github/workflows/lsan_suppressions.supp"
+            "extra_env_vars": "RERUN_USE_ASAN=1 LSAN_OPTIONS=suppressions=.github/workflows/lsan_suppressions.supp",
+            "pr_ci": true
         }
     ]
 }

.github/workflows/reusable_checks_cpp.yml

@@ -60,6 +60,7 @@ jobs:
     needs: matrix_prep
     strategy:
       matrix: ${{ fromJson(needs.matrix_prep.outputs.MATRIX) }}
+    if: ${{ github.event_name != 'pull_request' }} || ${{ matrix.pr_ci != false }}
     runs-on: ${{ matrix.runs_on }}
     steps:
       - uses: actions/checkout@v4