:alembic: add C++20 build

.github/workflows/ubuntu.yml

diff --git a/.github/workflows/ubuntu.yml b/.github/workflows/ubuntu.yml
index 58fdbd8..711cf10 100644
--- a/.github/workflows/ubuntu.yml
+++ b/.github/workflows/ubuntu.yml
@@ -54,7 +54,7 @@
           sudo apt install gcc-10 g++-10
         shell: bash
       - name: cmake
-      run: cmake -S . -B build -D CMAKE_BUILD_TYPE=Debug -DJSON_BuildTests=On -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_STANDARD_REQUIRED=ON
+        run: cmake -S . -B build -D CMAKE_BUILD_TYPE=Debug -DJSON_BuildTests=On -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_STANDARD_REQUIRED=ON
         env:
           CC:   gcc-10
           CXX:  g++-10