squash! squash! Don't require Doxygen from Conan if there's a system …

…install.

checkpoint

.github/workflows/library.yml

@@ -95,7 +95,7 @@ jobs:
         if: startsWith(matrix.config.os, 'ubuntu')
         run: |
           sudo apt-get update
-          sudo apt-get install --yes cmake ninja-build
+          sudo apt-get install --yes cmake ninja-build doxygen
           ninja --version
           cmake --version
           g++ --version