linux.yml: Downgrade to GCC 12

.. because ubuntu-22.04 suddenly dropped GCC 13

Author: hartwork

.github/workflows/linux.yml

@@ -13,13 +13,13 @@ env:
 jobs:
   linux:
     name: Build and test (Linux)
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     strategy:
       fail-fast: false
       matrix:
         include:
-          - cc: gcc-13
-            cxx: g++-13
+          - cc: gcc-12
+            cxx: g++-12
             clang_major_version: null
             clang_repo_suffix: null
           - cc: clang-18