Specify the OS version

'ubuntu-latest' doesn't have clang-12 in it anymore

Author: Andras Fekete

.github/workflows/multi-compiler.yml

@@ -12,19 +12,26 @@ jobs:
         include:
           - CC: gcc-9
             CXX: g++-9
+            OS: ubuntu-latest
           - CC: gcc-10
             CXX: g++-10
+            OS: ubuntu-latest
           - CC: gcc-11
             CXX: g++-11
+            OS: ubuntu-latest
           - CC: gcc-12
             CXX: g++-12
+            OS: ubuntu-latest
           - CC: clang-12
             CXX: clang++-12
+            OS: ubuntu-20.04
           - CC: clang-13
             CXX: clang++-13
+            OS: ubuntu-latest
           - CC: clang-14
             CXX: clang++-14
-    runs-on: ubuntu-latest
+            OS: ubuntu-latest
+    runs-on: ${{ matrix.OS }}
     # This should be a safe limit for the tests to run.
     timeout-minutes: 4
     steps: