Add Ubuntu 20.04 build.

.github/workflows/llvm.yml

@@ -9,6 +9,7 @@ jobs:
       fail-fast: false
       matrix:
         config:
+          - { os: ubuntu-20.04, platform: x64, cxx: g++-10, cc: gcc-10 }
           - { os: ubuntu-22.04, platform: x64, cxx: g++-11, cc: gcc-11 }
           - { os: macos-11, platform: x64, cxx: clang++, cc: clang }
           - { os: macos-12, platform: x64, cxx: clang++, cc: clang }