ci: suppress clang 6 + cxx20

Since the main branch that passed the same check 9 days ago also fails
with clang-6 and C++20 because of the same error, "undefined reference
to allocator_traits<char>::allocate". It could be a change in upstream
and since others (e.g. gcc) works well, I suppress the setting at this
moment.

.github/workflows/main.yml

@@ -58,6 +58,7 @@ jobs:
             - {compiler: '3.9', standard: '20'}
             - {compiler: '4.0', standard: '20'}
             - {compiler: '5.0', standard: '20'}
+            - {compiler: '6.0', standard: '20'}
     steps:
       - name: Checkout
         uses: actions/checkout@v2