The CI `build-werror` plan turns some warnings into error according to
what is defined by the cmake `ENABLE_WERROR` option, and only applies to
the default target built with Clang.
This diffs turns that build plan into a clang-10 build plan instead, and makes
the CI use the -Werror flag over all the regular builds. This will allow for preventing
warnings to get introduced for clang-8, clang-10 and gcc-8, with various flag
configurations.