github: Fix outdated clang9 config name

.github/workflows/build.yml

@@ -111,7 +111,7 @@ jobs:
             image: ubuntu-20.04,
             arch: x86_64,
             os: ubuntu,
-            compiler: clang9,
+            compiler: clang10,
             cc: clang-10,
             cxx: clang++-10
           }