Disable clang-12 build in github actions.

Workaround multi-day github actions flakiness:

E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/universe/l/llvm-toolchain-12/libclang-12-dev_12.0.0-3ubuntu1~20.04.4_amd64.deb  404  Not Found [IP: 52.252.75.106 80]

et al.

.github/workflows/ci-cpu-linux.yml

@@ -32,10 +32,11 @@ jobs:
             cc: "clang-11",
             cxx: "clang++-11",
           }
-        - {
-            cc: "clang-12",
-            cxx: "clang++-12",
-          }
+# WAR github actions flakiness
+#        - {
+#            cc: "clang-12",
+#            cxx: "clang++-12",
+#          }
 
       fail-fast: false